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 [2] 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). See also: * [1] http://kb.mozillazine.org/Session_Restore * [2] http://kb.mozillazine.org/sessionstore.js