Rename recovery.jsonlz4 to recovery-backup.jsonlz4.Open the sessionstore-backups folder of the profile.Open the profile folder and rename the current session file sessionstore.jsonlz4 to sessionstore-backup.jsonlz4.Make sure no instance of the browser is running. Mozilla changed the format of session backup files and that means that the previous way of restoring sessions needed updating. Use these if previous.jsonlz4 fails to do the trick. upgrade.jsonlz4-'number' - previous session backups, usually created during upgrades of the browser.previous.jsonlz4 - the last session backup that Firefox created.This is the file that you want to try to restore first (unless it has a very low size).
