I've had that problem, yeah... One new thing to look for though, Firefox v2 (I've been using it since the first 2.x beta) has a built-in spell checker that only comes on when it sees that you're typing in a text-box like this forum uses. It underlines the misspelled words as you type, and seems to be doing a good job of it.
I had a similar add-on for IE years ago that worked great, this is the first one for Firefox that really works well - and it's built in! /ubbthreads/images/graemlins/smile.gif