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