Right, this is one of the reasons people like to gripe and call Firefox "not a Mac program".
There is a second-best option. Firefox 2 beta has inline spell checking. It uses its own spell checker with its own dictionary, not the OS X stuff. Of course, this is useless if you rely on other plugins that aren't compatible with the beta.
Also, Camino development versions use the same checker as Firefox 2.
Works fairly well with 1.5.0.x, but I agree with the above posters that Firefox 2.0b2 is much, much better, and the built-in spell checker is better than SpellBound. (Though installing new languages is a bit of a hassle, for now.)