previous.js (cleanBackup: copy of sessionstore.js from previous session that was loaded successfully)
recovery.js (latest version of the sessionstore written during runtime)
recovery.bak (previous version of the sessionstore written during runtime)
upgrade.js-<build_id> (backup created during an upgrade of Firefox)

In case you use "Clear history when Firefox closes" or otherwise clear history. *do not clear the Browsing History *Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings *https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history *Clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data. ---- You can check for problems with the sessionstore.js file in the Firefox profile folder that stores session data. When sessionstore.js is deleted you lose App Tabs and Tab Groups and open tabs and you will have to recreate them, so if necessary make a note or bookmark them. You can use this button to go to the current Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox ---- Firefox uses the sessionstore.js file in the profile folder to store session data, but this file is only present when Firefox is closed. The sessionstore.js file is created from recovery.js when you close Firefox and is removed and copied to previous.js when you start Firefox to make is possible to restore the session at any time. Backup session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data. You can find these files in the sessionstore-backups folder: previous.js (cleanBackup: copy of sessionstore.js from previous session that was loaded successfully) recovery.js (latest version of the sessionstore written during runtime) recovery.bak (previous version of the sessionstore written during runtime) upgrade.js-<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.js to replace the current file (make a backup copy of the current sessionstore.js). See also: *http://www.ghacks.net/2014/06/27/mozilla-launches-improved-session-restore-firefox-33/