This can be a problem with the file [http://kb.mozillazine.org/sessionstore.js sessionstore.js] and sessionstore.bak in the [http://kb.mozillazine.org/Profile_folder_-_Firefox Profile Folder] Delete the files sessionstore.js and sessionstore.bak and any existing files sessionstore-##.js with a number in the left part of the name like sessionstore-1.js to prevent Firefox from restoring a previous session. See http://kb.mozillazine.org/Session_Restore