These two add-ons/programs seem to be causing the majority of startup crashes with Firefox 8.
1/ If you have the addon Roboform, be sure you have the latest version v7.6.2 because the previous versions of that addon cause crashes of FF
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=691271
You can delete the addon folder in your profile which is {22119944-ED35-4ab1-910B-E619EA06A115} (Roboform GUID).
2/ If you have the program Rapport from Trusteer Ltd (security tool provided by banks etc to secure user transactions), remove it. It is very crashy.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=699776