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 ---- * Use Restore or Maximize in the right-click context menu of the Taskbar icon to set focus to that Firefox window if you do not see the Firefox window. * Open the system menu of that Firefox window via Alt+Space and see if you can move and resize that window (use the keyboard cursor keys). If that works then first close all other open Firefox windows and then close Firefox via "File > Exit/Quit" to save that setting. You can also check for problems with the localstore.rdf file. *http://kb.mozillazine.org/Corrupt_localstore.rdf