I've long seen a problem with seamonkey when where you try to restore
a session, it starts to do it but then abruptly unmaps the main
browser window and (often) goes into an infinite loop. It turns out
that if you open additional browser windows before hitting restore,
sometimes you can avoid this problem... and usually the additional
windows remain operational, which allows you to discover that it
thinks the window(s) with your session exist, but you can't get to
them.
Does this match the behavior you're seeing? It would not surprise me
if firefox had the same underlying problem and for some reason it's
only just materialized.