v3.25.1

Balance Sheet Components - Schedule of Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Balance Sheet Related Disclosures [Abstract]
Prepaid insurance	$ 87	$ 280
Prepaid professional fees	0	211
Other	51	53
Total Other Assets	$ 138	$ 544

X

- Definition

Prepaid insurance noncurrent.

+ References

No definition available.

+ Details

Name:	fbrx_PrepaidInsuranceNoncurrent
Namespace Prefix:	fbrx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Prepaid professional fees noncurrent.

+ References

No definition available.

+ Details

Name:	fbrx_PrepaidProfessionalFeesNoncurrent
Namespace Prefix:	fbrx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- References

No definition available.

+ Details

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

X

- Definition

Amount of other miscellaneous assets expected to be realized or consumed after one year or normal operating cycle, if longer.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of noncurrent assets classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(17))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

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