Make sure you close the browser using; Windows: Menu > File > Exit Mac: Menu > Firefox > Quit Firefox Linux: Menu > File > Quit Some users have reported issues when closing the browser using the X upper-right of the window. You may have corrupt '''sessionstore''' [v56] '''sessionstore.jsonlz4''' file(s). Delete all '''sessionstore*''' files and the '''sessionstore-backups''' folder. '''Don't delete the files''' if you need to rescue any data from them, just move them out of the profile folder to some location where Firefox doesn't look for them. You can try to read out their contents using this tool: https://www.jeffersonscher.com/res/scrounger.html Type '''about:support'''<enter> in the address box. Under '''Troubleshooting Information''' on the left side, you will see '''Application Basics. ''' Under this find '''Profile Folder. ''' To its right press the button '''Show''' or '''Open Folder. ''' This will open your file browser to the current Firefox profile. Now '''Close Firefox. ''' Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder '''Linux: ''' Under the page logo on the left side, you will see '''Application Basics.''' Under this find '''Profile Directory. ''' To its right press the button '''Open Directory. ''' Locate the above file. Then rename or delete it. Restart Firefox.