I have just installed the new firefox 3 beta 4. I have been using betas of Firefox3 for a while, and have been enthusiastic about the release, but always had to keep a Firefox 2 copy around for viewing MathML. Without having tested this extensively, I would say that in beta 4, the level of MathML support is up to the level of Firefox 2, which allows me to make the transition to Firefox 3 fully. For some reason I do not fully understand, it seems that the font problems I was having with FF2 have also gone away.
To test FF3, I have looked at our MathML version of the Cornell eprint arXiv and the results are really impressive.