When Firefox closes then pinned tabs are stored as session data in sessionstore.jsonlz4 in the Firefox profile folder. You might have a recent backup in the sessionstore-backups folder. Backup the session files in the sessionstore-backups folder in the Firefox profile folder to make sure you do not lose possible important session data. Do NOT close Firefox when Firefox is already running. *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data You will normally find these files in the sessionstore-backups folder: *previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully) *recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime) *recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime) *upgrade.jsonlz4-<build_id> (backups created during a Firefox update) You can copy a file with Firefox closed from the sessionstore-backups folder to the main profile folder and rename the file to sessionstore.jsonlz4 to replace the current file. *make sure to backup the current sessionstore.jsonlz4 You can look at this tool to inspect a compressed jsonlz4 sessionstore file. This tool works locally, no uploading is done. *https://www.jeffersonscher.com/res/scrounger.html