One of my systems updated and restarted with 2.0 just fine.
My main system did not update automatically, and was running Firefox 1.5.0.09.
I hit the Update button and it upgraded to 1.5.0.10, not 2.0.
No direct answers that I saw quickly on the Mozilla forums, so I uninstalled and jumped to majorgeeks.com for a fresh install.
So did you upgrade to Version 2.0.0.2 then??