Backup the session files in the sessionstore-backups folder in the Firefox profile folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running. 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> (backup created during an upgrade of Firefox) You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.jsonlz4 to replace the current file with Firefox closed. *make sure to backup the current sessionstore.jsonlz4 You can try to restore a recent upgrade.jsonlz4-<build_id> when available or use a utility to browse Windows System Restore points to see if you can find a recent sessionstore.jsonlz4 file. You can look at this tool to inspect a compressed jsonlz4 sessionstore file. This tool works locally, no uploading done. *https://www.jeffersonscher.com/res/scrounger.html