You may have corrupt '''sessionstore''' file(s). Delete all '''sessionstore*''' files and the '''sessionstore-backups''' folder. Type '''about:support'''< enter > in the address bar. Under the page logo on the left side, you will see '''Application Basics. ''' Under this find '''Profile Folder. ''' To its right press the button '''Show Folder. ''' This will open your file browser to the current Firefox profile. Now '''Close Firefox. ''' Locate the above files and folder. Then rename or delete them. Restart Firefox.