Install Nightly Tester Tools, and in your addons area, if you have any addons that can't run because of compatibility issues, there should be a button in the corner of the addons box next to the Find Updates button called something like Force Compatibility. Press that and restart FF, and now all of your addons should work.
If you still can't figure it out, take a look at this video:
The guy's using FF 2, but it's still basically the same procedure in FF 3.5.