A Vague memory from the distant past suggests that the thing to try is to move aside the .mozilla directory, and allow the new firefox to start in a completely clean environment.
I kind of recall that continuous startup problems, which only affect some users, tend to be related to data somewhere in there (I have no idea which data) something isn't the way the new version expects it, and all kinds of problems ensue. If your old version is still working, you should probably have it export everything possible that you can first (bookmarks, etc) so that if the new one works in a clean environment you can import as much of what you'd hope to retain as possibble. Of course, this might all be a red herring, so don't destroy the old .mozilla directory, just move it, so it coudl be replaced if this turns out not to be the problem (also so that it can be replaced if you decide to revert to the old version.) kre