The primary backup file is recovery.js now, the secondary backup file recovery.bak. Note that sessionstore.js is only shown when Firefox is not open. While sessionstore.js is still stored in the main Firefox profile folder, all other files are now stored in sessionstore-backups instead. Mozilla has improved the Session Restore feature in Firefox 33. May still be useful for old versions of Firefox, and Firefox-based browsers such as Pale Moon. If it does not, press the Alt-key on your keyboard and select History > Restore Previous Session. Firefox might pick up the session automatically.Paste the copied file to the root folder.Go back to the root directory of the Firefox profile.Copy the session restore backup that you want to restore, e.g.Rename recovery.backlz4 to recovery-backup.backlz4.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).

New firefox restore previous session