previous.jsonlz4 is never updated during the current session as this is a copy of sessionstore.jsonlz4 from when Firefox was last started (i.e. files restored when you opened this session). The recovery files will/should be updated when you make changes to open tabs although there might be a delay if you make many changes in a short time. 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