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 file. Then rename or delete it. Restart Firefox.

Found two such files: a backup and a .js. Deleted both with FF closed. Restarted open 5 windows, closed FF. restarted, and only two of the 5 held content. The first (home page) and the last one I opened. I will repeat, just to be sure, but I'm sure. :-) Thx.

I did see, and did not delete, session-state.json and sessionCheckpoints.json