Not sure why but firefox seemed to lose it's session restore feature for me. Maybe something to do with having tab mix plus installed prior to the 2 -> 3 upgrade.
Anyhow here is how to restore session saving.
Enter about:config in the address bar and say yes to the behave yourself warning!
Search down for "browser.sessionstore.resume_session_once" and change the setting to true.
and restart ...
That's it.