You can make a backup copy of the sessionstore.js in the Firefox Profile Folder with Firefox closed. Then you can restore that copy to recover those lost tabs. *http://kb.mozillazine.org/sessionstore.js *Help -> Probleemoplossingsinformatie -> Profielmap -> Map tonen What is the startup setting? *Extra -> Opties -> Algemeen > Opstarten *https://support.mozilla.org/kb/Options+window+-+General+panel ---- It is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder. Delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox Profile Folder. *Help > Troubleshooting Information > Profile Directory: Open Containing Folder *http://kb.mozillazine.org/Profile_folder_-_Firefox Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them). *http://kb.mozillazine.org/Multiple_profile_files_created