It's rather unfair to judge Firefox by its current state (re. "crashing every 10 minutes"). Remember it's a by no means a final release. When a completed Firefox is ready, and there's a completed O3 about, then I would hope both would be pretty stable.
Unless Firefox does eventually become a much more responsive, fully RISC OS integrated browser, then there will always be room for both it and a native one, be it NetSurf or O3 - whichever does best, with FF providing a useful resource when those others aren't up to the job. Which, IIRC, is what Peter's said all along.