That's interesting, I've never had issues getting old sessions back. You could try recovering it via "History > Recently closed windows", it should be there usually.
You can probably blame the web apps Im using for not saving state... But honestly I shouldn't be banned from opening tabs until I reboot the browser.
I've never had this happen personally. What OS is this? I use Firefox on OSX installed through Homebrew, so I have to manually update it with brew. Firefox has never asked me to update.
Ubuntu bionic. Latest default Firefox without any special enterprise stuff or whatnot.
It happens when you update firefox out from under itself. In other words, don't apply apt updates for firefox until you're ready to restart it.