Have a look. https://support.mozilla.org/en-US/questions/1415643 https://support.mozilla.org/en-US/questions/1437447#answer-1632326 Going forward, '''I HIGHLY''' recommend you make a back up of the folder '''sessionstore-backups'''. see screenshot Every now and then make a copy because I'm seeing that when the session fails to restore, the files are deleted and you only have the recovery files(see 2nd screenshot) that are new and there's nothing to restore. The 2nd screenshot shows the new files on the left after the session failed to restore. The right side of the 2nd pic showing the copy that I made a few days before.