You could try pinning tabs you want to keep before restarting. I do the same if I really want to be sure. Pinned tabs generally survive restarts, even during updates.

I had this exact scenario (asking me to restart after an update) happen to me yesterday on pop os which is Ubuntu based. And I've definitely seen it other times too. Having said that, it was absolutely no big deal whatsoever.

I don't remember seeing the other issue you mention. I've literally had some tabs open several months. Have you tried looking for the "restore tabs" option in the History menu when this happens? Afaik that's available even if you don't have any option set to save your tabs

If that happens, you should recover from a snapshot... I never experienced this unless the profile folder was damaged or lost.

If you are okay with trusting an extension, I use Tab Session Manager, it takes snapshots of all your open windows and tabs, and it can restore them when, for example, you forget to restore the previous session before closing Firefox, and overwrite the previous session of dozens of tabs, with the current session that you don't care about.

I usually just rely on the built-in restore, but having the backup just in case is such a relief to have on those occasions when "reload last session" is grayed out.