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.

When Firefox opens, is

History > Restore Previous Session

available?

If so, hmm, I wonder why Firefox is not restoring it?

If not, that may be because:

(1) Firefox already restored the file. Does the History menu list any previously closed windows you could re-open?

(2) Firefox did not save session history because it is set to clear history when it closes.

(3) Another program removed the file (e.g., cleaning software).

FredMcD said

You may have corrupt sessionstore 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

I ended up uninstalling and reinstalling and it seemed to do the trick. Thanks for the help!

That was very good work. Well done.