Firefox may not be closing properly if the previous session doesn't get reloaded. Use "Firefox/File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit") to close Firefox if you are currently doing that by clicking the close X on the title bar. Make sure that you do not use Clear History to clear the <i>Browsing History</i> when you close Firefox. *Tools > Options > Privacy: History: [X] Clear history when Firefox closes > Settings *https://support.mozilla.org/kb/Clear+Recent+History ----- It also 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 [3] 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 *[3] http://kb.mozillazine.org/Multiple_profile_files_created