Income Taxes - Tax Credits and Carryforwards (Details) $ in Thousands	Dec. 28, 2024 USD ($)
California
Operating Loss Carryforwards [Line Items]
California research credits	$ 61,491
Singapore
Operating Loss Carryforwards [Line Items]
Singapore net operating loss carryforwards	3,936
Federal
Operating Loss Carryforwards [Line Items]
Federal research and development tax credit	12,107
Foreign tax credit carryforwards	752
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 238,852

Deferred Tax Assets, Tax Credit Carryforwards, Foreign, Subject to Expiration

No definition available.

Name:	form_DeferredTaxAssetsTaxCreditCarryforwardsForeignSubjectToExpiration
Namespace Prefix:	form_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Deferred Tax Assets, Tax Credit Carryforwards, Research, Subject to Expiration

+ References

No definition available.

+ Details

Name:	form_DeferredTaxAssetsTaxCreditCarryforwardsResearchSubjectToExpiration
Namespace Prefix:	form_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards that are not subject to expiration dates.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-6

+ Details

Name:	us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards that are subject to expiration dates.

+ References

+ Details

Name:	us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible research tax credit carryforwards.

+ References

+ Details

Name:	us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-3

+ Details

Name:	us-gaap_OperatingLossCarryforwardsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_IncomeTaxAuthorityNameAxis=us-gaap_CaliforniaFranchiseTaxBoardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeTaxAuthorityNameAxis=us-gaap_InlandRevenueSingaporeIRASMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeTaxAuthorityAxis=us-gaap_InternalRevenueServiceIRSMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeTaxAuthorityAxis=us-gaap_StateAndLocalJurisdictionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: