You would need an older copy of sessionstore.js from yesterday. You can check in Windows Explorer if there is a previous version of the sessionstore.js file in the profile folder available. *right-click: Properties -> Previous Versions *http://windows.microsoft.com/en-us/windows-8/how-use-file-history You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page. *Help -> Troubleshooting Information -> Profile Directory:<br>Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder *http://kb.mozillazine.org/Profile_folder_-_Firefox ---- If it happens again then do not close Firefox, but make a backup of the files in the sessionstore-backups folder Backup the session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running. You will normally 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/