You can check for problems with sessionstore.js file(s) in the Firefox profile folder that store session data. *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Multiple_profile_files_created Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost and you will have to recreate them (make a note or bookmark them if possible). See also: *https://support.mozilla.org/kb/restore-previous-session *http://kb.mozillazine.org/browser.sessionstore.resume_from_crash Make sure that you do not run Firefox in (permanent) Private Browsing mode (Never remember history). *https://support.mozilla.org/kb/Private+Browsing *Tools > Options > Privacy > Firefox will: "Use custom settings for history" *Deselect: [ ] "Always use Private Browsing mode"