The main advantage seems to be that some extentions are now being made only for Firefox, or have abandoned Suite support. And Deepest Sender seems less buggy on Firefox (the link button actually works, for example). On the other hand, my preferred skin, Pinball, has not yet been ported to Firefox and the author seems disinclined to make that a priority.
I've only just started using it, and I'll continue to experiment with it to see if my opinion changes. But right now my opinion is that Firefox's popularity relative to the Suite has more to do with marketing and having a sexier name than actually being better.