In case you are using "Clear history when Firefox closes": *do not clear the Browsing History *Edit > Preferences > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings *https://support.mozilla.org/kb/remove-browsing-search-and-download-history Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data. ---- You can check for problems with the sessionstore.js and sessionstore.bak files in the Firefox profile folder that store session data. Rename (or 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: 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).

Thanks, Problem solved