Every so often they manage to break what wasn't broke, so I've been sticking with 1.5.0.6.
Get all your themes and extensions and tweak the browser the way you like it, and then get this program. (It might not backup all variations of themes and extensions, but it has for my particular setup).
I installed the latest Fedora yesterday, restored my .mozilla directory, and I was surfing again just like I was on Friday.