You can check for problems with the sessionstore.js file in the Firefox profile folder that store session data. *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Multiple_profile_files_created Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost and you will have to recreate them (make a note or bookmark them if possible). ---- The sessionstore.js file in the profile folder is still used to store session data, but is only present when Firefox is closed. The sessionstore.js file is created from recovery.js when you close Firefox and removed when you start Firefox and copied to previous.js. The sessionstore.bak file is no longer used and has been replaced by the recovery and previous files that you find in the sessionstore-backups folder. The upgrade files in this folder with the build ID appended are created when Firefox updates.