Post a screenshot of the files in your sessionstore-backup folder that's in your profile folder. see screenshots in link below. https://support.mozilla.org/en-US/questions/1477659#answer-1693216

As requested, attached is the screenshot of the files in my sessionstore-backup

Oh yeah... It's a wrap. As you can see from the size of the files that there is nothing to go back to. See screenshot of my folder. You can see from the number of flies in my sessionstore-backups and the copy of the folder that I made on 11/28. You see less files now in my sessionstore-backups folder which shows that files were erased from the folder. That's why I make copies of the folders about once a week.

Is there another profile? Just go up 1 folder and see if there's another profile. If so, then look for the sessionstore-bakups folder in the other possible profile folders.

Here's a view from my Nightly profile. You can see the the session folder has the same number of files as the copy I made back in November.

There is another profile but its empty. Attached is a snippet of how the Firefox folder looks like.

Do you normally sign in under the Windows user "TEMP" or has Windows done something weird at the user level?

I sign in with a different user, so, it looks like Windows has done something weird at the user level.