
<PAGE>   1
     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON JULY 27, 1999.
                                                      REGISTRATION No. 333-18691



                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549


                         POST-EFFECTIVE AMENDMENT NO. 2
                                       TO
                                    FORM S-8
             REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933


                                 CHEMFIRST INC.
             (Exact name of registrant as specified in its charter)

          MISSISSIPPI                                          64-0679456
(State or other jurisdiction of                             (I.R.S. Employer
incorporation or organization)                           Identification Number)


                                700 NORTH STREET
                           JACKSON, MISSISSIPPI 39202
               (Address of Principal Executive Offices) (Zip Code)




   CHEMFIRST INC. 401(k) SAVINGS AND EMPLOYEE STOCK OWNERSHIP PLAN AND TRUST
                            (Full title of the plan)


                                JAMES L. MCARTHUR
                                    SECRETARY
                                 CHEMFIRST INC.
                                700 NORTH STREET
                           JACKSON, MISSISSIPPI 39202
                                 (601) 948-7550
     (Name and address, including zip code, and telephone number, including
                        area code, of agent for service)




  In addition, pursuant to Rule 416 under the Securities Act of 1933, as
amended, this Registration Statement also covers shares of Common Stock of
ChemFirst Inc. issuable to prevent dilution resulting from stock splits, stock
dividends or similar transactions.


<PAGE>   2
        This Post-Effective Amendment No. 2 on Form S-8 amends and restates that
certain Registration Statement No. 333-18691 on Form S-8 filed December 24,
1996, as amended by that certain Post-Effective Amendment No. 1 filed on January
17, 1997 relating to the ChemFirst Inc. 401(k) Savings Plan, as amended and
merged, the ChemFirst Inc. 401(k) Savings and Employee Stock Ownership Plan and
Trust. Accordingly the contents of the Registration Statement are incorporated
in this Post-Effective Amendment No. 2 by reference.


                                     PART I
              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

        The information specified by Items 1 and 2 of Part I of Form S-8 is
omitted from this filing in accordance with provisions of Rule 428 under the
Securities Act of 1933 (the "Securities Act") and the introductory Note to Part
I of Form S-8.


                                     PART II
               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE.

        The documents set forth below are incorporated by reference in this
Registration Statement. All documents subsequently filed by ChemFirst Inc.
("ChemFirst") and the ChemFirst Inc. 401(k) Savings and Employee Stock Ownership
Plan and Trust pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Securities
Exchange Act of 1934 (the "Exchange Act"), prior to the filing of a
post-effective amendment which indicates that all securities offered have been
sold or which deregisters all securities then remaining unsold, shall be deemed
to be incorporated by reference in this Registration Statement and to be part
hereof commencing on the respective dates on which such documents are filed. Any
statement contained in a document incorporated or deemed to be incorporated by
reference herein shall be deemed to be modified or superseded for purposes of
this Registration Statement to the extent that a statement contained herein or
in any other subsequently filed document which also is or is deemed to be
incorporated by reference herein modifies or supersedes such statement. Any such
statement so modified or superseded shall not be deemed, except as so modified
or superseded, to constitute a part of this Registration Statement.

        (1)     ChemFirst's Annual Report on Form 10-K for the year ended
                December 31, 1998;

        (2)     The Annual Report on Form 11-K for the ChemFirst Inc. 401(k)
                Savings and Employee Stock Ownership Plan and Trust for the plan
                year ended December 31, 1998;

        (3)     ChemFirst's quarterly report on Form 10-Q for the quarterly
                period ended March 31, 1999;

        (4)     All other reports filed with the Securities and Exchange
                Commission pursuant to Section 13(a) or 15(d) of the Exchange
                Act since the end of the fiscal year covered by the reports in
                (1) and (2) above; and

        (5)     The description of the Common Stock which is contained in
                ChemFirst's Registration Statement on Form 8-A dated December 9,
                1996, filed pursuant to Section 12 of the Exchange Act, and all
                amendments thereto and reports which have been filed for the
                purpose of updating such description.

ITEM 4. DESCRIPTION OF SECURITIES.

        Not Applicable.

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL.

        Not Applicable.


                                      II-1
<PAGE>   3

ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

        Subarticle E of Article 8 of the Mississippi Business Corporation Act
        ("MBCA") empowers a Mississippi corporation to indemnify against
        liability an individual who is made a party to any threatened, pending
        or completed action, suit or proceeding, whether civil, criminal,
        administrative or investigative, formal or informal (a "Proceeding"),
        because such person is or was a director of the corporation. To be
        eligible for indemnification, the director must have conducted himself
        in good faith and in a manner such person reasonably believed to be in
        or not opposed to the best interests of the corporation and, with
        respect to any criminal action or proceeding, had no reasonable cause to
        believe such person's conduct was unlawful. Liability indemnified
        against includes the obligation to pay a judgment, settlement, penalty,
        fine or reasonable expenses incurred with respect to a Proceeding. The
        MBCA precludes a corporation from indemnifying a director in connection
        with a Proceeding by or in the right of the corporation in which the
        director was adjudged liable to the corporation or in connection with
        any other Proceeding charging improper personal benefit to a director,
        whether or not involving action in the director's official capacity, in
        which the director was adjudged liable on the basis that personal
        benefit was improperly received by the director.

        Subarticle E further provides that if a director is wholly successful,
        on the merits or otherwise, in the defense of any Proceeding to which he
        was a party because he is or was a director, the corporation must
        indemnify him against reasonable expenses incurred in connection with
        the Proceeding. Also, a court may order a company to indemnify a
        director if it determines the director is fairly and reasonably entitled
        to indemnification in view of all of the relevant circumstances.
        Subarticle E also allows corporations to indemnify officers, employees
        or agents to the same extent as directors, and provides for mandatory or
        court-ordered indemnification for these persons as described above.
        Finally, the MBCA allows corporations to purchase and maintain insurance
        on behalf of directors, officers, employees or agents against liability
        asserted against or incurred by them in that capacity or arising from
        their status as such, whether or not the corporation would have the
        power to indemnify such person against liability under Subarticle E.

        ChemFirst's Bylaws provide for indemnification of ChemFirst's officers
        and directors to the fullest extent allowed by Mississippi law and
        further permit such indemnification with respect to other employees and
        agents. ChemFirst entered into indemnification agreements with certain
        of its officers and its directors. The effect of these agreements is to
        add a contractual right of indemnification to the indemnification rights
        otherwise granted by law.

        ChemFirst maintains directors' and officers' liability insurance which
        protects each director and officer from certain claims and suits,
        including shareholder derivative suits, even where the director may be
        determined to not be entitled to indemnification under the MBCA and
        claims and suits arising under the Securities Act. The policy may also
        afford coverage under circumstances where the facts do not justify a
        finding that the director or officer acted in good faith and in a manner
        that was in or not opposed to the interests of ChemFirst.

        The foregoing represents a summary of the general effect of the MBCA,
        ChemFirst's Bylaws, indemnity agreements, and directors' and officers'
        liability insurance coverage for purposes of general description only.

ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED.

        Not Applicable.

ITEM 8. EXHIBITS.

        In lieu of certain exhibit requirements, ChemFirst will submit or has
        submitted the Plan and any amendment thereto to the Internal Revenue
        Service ("IRS") in a timely manner and has made or will make all changes
        required by the IRS in order to continue to qualify the Plan under
        Section 401 of the Internal Revenue Code.

        4.1*    Amended and Restated Articles of Incorporation of ChemFirst are
                incorporated by reference to Exhibit 3.1 of ChemFirst's S-1
                Registration Statement (file number 333-15789).


                                      II-2
<PAGE>   4

        4.2*    Bylaws of ChemFirst, as amended, are incorporated by reference
                to Exhibit 4.3 of ChemFirst's S-8 Registration Statement (file
                number 333-69965).

        4.3*    Rights Agreement dated as of October 30, 1996, by and between
                ChemFirst and KeyCorp Shareholder Services, Inc. is incorporated
                by reference to Exhibit 4 of ChemFirst's S-1 Registration
                Statement (file number 333-15789).

        4.4*    First Amendment to Rights Agreement by and among ChemFirst,
                KeyCorp Shareholder Services, Inc. and the Bank of New York is
                incorporated by reference to Exhibit 4.5 to ChemFirst's S-8
                Registration Statement (file number 333-69965).

        4.5*    ChemFirst's 401(k) Savings Plan is incorporated by reference to
                Exhibit 4.4 of ChemFirst's S-8 Registration Statement (file
                number 333-18691).

        4.6     ChemFirst Inc. 401(k) Savings and Employee Stock Ownership Plan
                and Trust (as amended and restated effective January 1, 1997;
                supersedes ChemFirst's 401(k) Savings Plan, previously filed as
                Exhibit 4.5).

        4.7     First Amendment to ChemFirst Inc. 401(k) and Employee Stock
                Ownership Plan and Trust.

        4.8*    Note Purchase Agreement between ChemFirst, State Farm Life
                Insurance Company and Nationwide Life Insurance Company is
                incorporated by reference to Exhibit 4(j) of ChemFirst's Annual
                Report on Form 10-K for fiscal year ended December 31, 1998.

        4.9*    ChemFirst Inc. 1998 Long-Term Incentive Plan is incorporated by
                reference to Appendix A to ChemFirst's Proxy Statement filed in
                connection with its May 27, 1998 annual meeting of stockholders.

        4.10    Second Amendment to ChemFirst Inc. 401(k) Savings and Employee
                Stock ownership Plan and Trust.

        5.3*    Determination letter dated April 24, 1996 from the IRS regarding
                ChemFirst's 401(k) Savings Plan is incorporated by reference to
                Exhibit 5.3 of ChemFirst's Registration Statement (file number
                333-18691).

        23.1    Consent of KPMG Peat Marwick LLP.

        24.1    Powers of Attorney is included on page II-5 herein.


* Previously filed.

ITEM 9. UNDERTAKINGS.

        ChemFirst herein undertakes:

        (1)     To file, during any period in which offers or sales are being
                made, a post-effective amendment to this Registration Statement:

                (i)     To include any prospectus required by Section 10(a)(3)
                        of the Securities Act;

                (ii)    To reflect in the prospectus any facts or events arising
                        after the effective date of this Registration Statement
                        (or the most recent post-effective amendment thereof)
                        which, individually or in the aggregate, represent a
                        fundamental change in the information set forth in this
                        Registration Statement. Notwithstanding the foregoing,
                        any increase or decrease in volume of securities offered
                        (if the total dollar value of securities offered would
                        not exceed that which was registered) and any deviation
                        from the low or high end of the estimated maximum
                        offering range may be reflected in the form of
                        prospectus filed with the Commission pursuant to Rule
                        424(b) if, in the aggregate, the changes in volume and
                        price represent no more than a 20 percent change in the
                        maximum aggregate offering price set forth in the
                        "Calculation of Registration Fee" table in the effective
                        registration statement;


                                      II-3
<PAGE>   5

                (iii)   To include any material information with respect to the
                        plan of distribution not previously disclosed in this
                        Registration Statement or any material change to such
                        information in the Registration Statement;

                provided, however, that paragraphs (1)(i) and (1)(ii) do not
                apply if the registration statement is on Form S-3, Form S-8 or
                Form F-3, and the information required to be included in a
                post-effective amendment by those paragraphs is contained in
                periodic reports filed with or furnished to the Securities and
                Exchange Commission by ChemFirst pursuant to Section 13 or
                Section 15(d) of the Exchange Act that are incorporated by
                reference in the Registration Statement.

        (2)     That, for the purpose of determining any liability under the
                Securities Act, each such post-effective amendment shall be
                deemed to be a new registration statement relating to the
                securities offered therein, and the offering of such securities
                at that time shall be deemed to be the initial bona fide
                offering thereof.

        (3)     To remove from registration by means of a post-effective
                amendment any of the securities being registered which remain
                unsold at the termination of the offering.

        (4)     That, for purposes of determining any liability under the
                Securities Act, each filing of ChemFirst's annual report
                pursuant to Section 13(a) or Section 15(d) of the Exchange Act
                (and, where applicable, each filing of an employee benefit
                plan's annual report pursuant to Section 15(d) of the Exchange
                Act) that is incorporated by reference in the Registration
                Statement shall be deemed to be a new registration statement
                relating to the securities offered therein, and the offering of
                such securities at that time shall be deemed to be the initial
                bona fide offering thereof.

        (5)     Insofar as indemnification for liabilities arising under the
                Securities Act may be permitted to directors, officers and
                controlling persons of ChemFirst pursuant to the foregoing
                provisions, or otherwise, ChemFirst has been advised that in the
                opinion of the Securities and Exchange Commission such
                indemnification is against public policy as expressed in the
                Securities Act and is, therefore, unenforceable. In the event
                that a claim for indemnification against such liabilities (other
                than the payment by ChemFirst of expenses incurred or paid by a
                director, officer or controlling person of ChemFirst in the
                successful defense of any action, suit or proceeding) is
                asserted by such director, officer or controlling person in
                connection with the securities being registered, ChemFirst will,
                unless in the opinion of its counsel the matter has been settled
                by controlling precedent, submit to a court of appropriate
                jurisdiction the question whether such indemnification by it is
                against public policy as expressed in the Securities Act and
                will be governed by the final adjudication of such issue.


                                      II-4
<PAGE>   6

                                   SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, Registrant
has duly caused this Post-Effective Amendment No. 2 to the Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Jackson, State of Mississippi on July 27, 1999.

                                         CHEMFIRST INC.


                                         By: /s/ J. Kelley Williams
                                             -----------------------------------
                                                 J. Kelley Williams
                                                 Chief Executive Officer

        Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.

        KNOW ALL MEN AND WOMEN BY THESE PRESENTS, that each person whose
signature appears below constitutes and appoints J. Kelley Williams and R.
Michael Summerford, and each of them, such individuals' true and lawful
attorneys-in-fact and agents, with full power of substitution and
resubstitution, for such individual and in his or her name, place and stead, in
any and all capacities, to sign any and all amendments (including post-effective
amendments) to this Registration Statement and any registration statement
related to the offering contemplated by this registration statement that is to
be effective upon filing pursuant to Rule 462(b) under the Securities Act of
1933, and to file the same, with all exhibits thereto, and all documents in
connection therewith, with the Securities and Exchange Commission, granting unto
said attorneys-in-fact and agents, and each of them, full power and authority to
do and perform each and every act and thing requisite and necessary to be done
in and about the premises as fully and to intents and purposes as he might or
could do in person, hereby ratifying and confirming all that said
attorneys-in-fact and agents, or any of them, or their substitute or
substitutes, may lawfully do or cause to be done by virtue hereof.

<TABLE>
<CAPTION>
              SIGNATURE                                     TITLE                                 DATE
              ---------                                     -----                                 ----
<S>                                          <C>                                              <C>
/s/ J. Kelley Williams                       Chairman of the Board of Directors,              July 27, 1999
------------------------------------         Chief Executive Officer (Principal
J. Kelley Williams                           Executive Officer)


/s/ R. M. Summerford                         President and Chief Operating Officer            July 27, 1999
------------------------------------
R. M. Summerford


/s/ Max P. Bowman                            Vice President, Finance and Treasurer
------------------------------------         (Principal Financial Officer)                    July 27, 1999
Max P. Bowman


/s/ Troy B. Browning                         Controller (Principal Accounting
------------------------------------         Officer)                                         July 27, 1999
Troy B. Browning


/s/ Richard P. Anderson                      Director                                         July 27, 1999
------------------------------------
Richard P. Anderson

/s/ Paul A. Becker                           Director                                         July 27, 1999
------------------------------------
Paul A. Becker
</TABLE>


                                      II-5
<PAGE>   7

<TABLE>
<S>                                          <C>                            <C>
/s/ James W. Crook                           Director                       July 27, 1999
------------------------------------
James W. Crook


/s/ Michael J. Ferris                        Director                       July 27, 1999
------------------------------------
Michael J. Ferris

/s/ James E. Fligg                           Director                       July 27, 1999
------------------------------------
James E. Fligg


/s/ Robert P. Guyton                         Director                       July 27, 1999
------------------------------------
Robert P. Guyton


/s/ Paul W. Murrill                          Director                       July 27, 1999
------------------------------------
Paul W. Murrill


/s/ William A. Percy, II                     Director                       July 27, 1999
------------------------------------
William A. Percy, II


/s/ Dan F. Smith                             Director                       July 27, 1999
------------------------------------
Dan F. Smith


/s/ Leland R. Speed                          Director                       July 27, 1999
------------------------------------
Leland R. Speed


/s/ R. Gerald Turner                         Director                       July 27, 1999
------------------------------------
R. Gerald Turner
</TABLE>


                                      II-6
<PAGE>   8

        Pursuant to the requirements of the Securities Act of 1933, the Plan has
duly caused this Registration Statement to be signed on its behalf by the
undersigned, thereunto duly authorized, in the City of Jackson, State of
Mississippi, on the 27th day of July, 1999.


                                      CHEMFIRST INC. 401(K) SAVINGS AND EMPLOYEE
                                      STOCK OWNERSHIP PLAN AND TRUST


                                      By: /s/ J. Steve Chustz
                                          --------------------------------------
                                          J. Steve Chustz
                                          Employee Benefits Committee


                                      By: /s/ William B. Kemp, Jr.
                                          --------------------------------------
                                          William B. Kemp, Jr.
                                          Employee Benefits Committee


                                      By: /s/ George M. Simmons
                                          --------------------------------------
                                          George M. Simmons
                                          Employee Benefits Committee


                                      By: /s/ R. M. Summerford
                                          --------------------------------------
                                          R. M. Summerford
                                          Employee Benefits Committee


                                      II-7
<PAGE>   9

                                INDEX TO EXHIBITS



<TABLE>
<CAPTION>
         EXHIBIT
         NUMBER            EXHIBIT
         ------            -------

         <S>      <C>
         4.1*     Amended and Restated Articles of Incorporation of ChemFirst
                  are incorporated by reference to Exhibit 3.1 of ChemFirst's
                  S-1 Registration Statement (file number 333-15789).

         4.2*     Bylaws of ChemFirst, as amended, are incorporated by reference
                  to Exhibit 4.3 of ChemFirst's S-8 Registration Statement (file
                  number 333-69965).

         4.3*     Rights Agreement dated as of October 30, 1996, by and between
                  ChemFirst and KeyCorp Shareholder Services, Inc. is
                  incorporated by reference to Exhibit 4 of ChemFirst's S-1
                  Registration Statement (file number 333-15789).

         4.4*     First Amendment to Rights Agreement by and among ChemFirst,
                  KeyCorp Shareholder Services, Inc. and the Bank of New York is
                  incorporated by reference to Exhibit 4.5 to ChemFirst's S-8
                  Registration Statement (file number 333-69965).

         4.5*     ChemFirst's 401(k) Savings Plan is incorporated by reference
                  to Exhibit 4.4 of ChemFirst's S-8 Registration Statement (file
                  number 333-18691).

         4.6      ChemFirst Inc. 401(k) Savings and Employee Stock Ownership
                  Plan and Trust (as amended and restated effective January 1,
                  1997; supersedes ChemFirst's 401(k) Savings Plan, previously
                  filed as Exhibit 4.5).

         4.7      First Amendment to ChemFirst Inc. 401(k) and Employee Stock
                  Ownership Plan and Trust.

         4.8*     Note Purchase Agreement between ChemFirst, State Farm Life
                  Insurance Company and Nationwide Life Insurance Company is
                  incorporated by reference to Exhibit 4(j) of ChemFirst's
                  Annual Report on Form 10-K for fiscal year ended December 31,
                  1998.

         4.9*     ChemFirst Inc. 1998 Long-Term Incentive Plan is incorporated
                  by reference to Appendix A to ChemFirst's Proxy Statement
                  filed in connection with its May 27, 1998 annual meeting of
                  stockholders.

         4.10     Second Amendment to ChemFirst Inc. 401(k) Savings and Employee
                  Stock Ownership Plan and Trust.

         5.3*     Determination letter dated April 24, 1996 from the IRS
                  regarding ChemFirst's 401(k) Savings Plan is incorporated by
                  reference to Exhibit 5.3 of ChemFirst's Registration Statement
                  (file number 333-18691).

         23.1     Consent of KPMG Peat Marwick LLP.

         24.1     Powers of Attorney is included on page II-5 herein.
</TABLE>

----------------
* Previously filed.

