Have you tried going to the special "about:support" URL and clicking the "Reset Firefox" button at the top right? This creates a new profile and imports all your existing bookmarks, history, saved passwords, etc. but resets preferences back to defaults and recreates all the databases from scratch. If there's any settings from old version of Firefox that are confusing the new version, or poorly-laid-out database files that can't be efficiently updated, this should clean them up.