Annoying indeed but at least on my machines I can always get them back manually if they don't show up automatically by going to the history menu and choose "Restore Previous Session". Hopefully this should work everywhere.

(Note: I'm trying to help, not place blame here. I won't blame anyone for not being aware of every power user trick but I hope to help more people become power users. Please do ask questions about Firefox, even if I'm moving to LibreWolf I still wish Firefox well and think many would be better off using them but just aren't aware : )