Did you just upgrade your install to 3.5.1 by any chance?!?!?
I upgraded my install at work today, and it borked all my extensions, et cetera.
The way I fixed it was by running "firefox -p" from a CLI and creating a new profile.
LoL!
This is the second time, in about 6 months, that a Firefox update has blitzed my install - once at work, and once at home.
Makes me wonder...