You may have corrupt '''sessionstore''' [v56] '''sessionstore.jsonlz4''' 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. ''' 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. '''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

Thank you! I will try it next time when it happens.