General tab in Preferences, you might want to set
When Firefox Starts to Show my Windows and Tabs from Last Time. It's very possible that Firefox 4 (which, as bruno077 reminded us, still is in beta) isn't registering the fact that it crashed correctly.
Also, make sure that
browser.sessionstore.resume_from_crash is set to
true and that
browser.sessionstore.max_resumed_crashes is greater than or equal to one in