been using FF3 for many month now and i must say it is much better than FF2 (my opinion, my preference) .. and about the add on, i've tried 3 beta version of FF3 and add-on update was fast .. in a matter of week usually, author of the add-on update his/her add-on to be compatible with the latest ver.

but depend on what add-on you were using ler .. unknown/rare one (last updated last year ) of course will have problem