Then it's just a mess of bookmarks instead of tabs. And maybe I was in the middle of doing something and decided that I didn't want to do it anymore right then but would eventually return to it, which could involve multiple tabs together.

But I did get an addon to help save and manage my tabs after an instance of closing my main window while having another browser window open (sometimes it's nice to pull a tab out to a seperate window so you can see two tabs at once), which would lose the full set of tabs in FF (other than the single one in the other window).

When the browser restarts, it doesn't automatically load all of those tabs until you click over to them (at least by default, I think there is a setting for it if you'd rather it did).