I suggest a tab grouping addon because there are scenarios where you can still lose your tabs with just session restore (eg if you open a new browser window, that often gets treated as a different session and it will only save the one closed last, so if you close the big tab one first and then close a single tab, you might find it only has that single tab saved when you load it up again).

I use Simple Tab Groups, which is set up to hide tabs from other groups until I switch to them, so it doesn't depend on what is going on in the active session (well, kinda, FF does provide a lot of the infra, like the loaded tabs from inactive groups are still loaded), but uses its own data to save/restore tabs that isn't just "save a flat list of tabs when window is closed".