Yes, I'd love for them to be as similar under the hood as possible. But last time I checked Fennec (admittedly an alpha build), I loved the double-tap to zoom. However, from a video I saw, it looked like this feature has been integrated into MicroB for the N900. I didn't like Fennec's lack of a scroll bar (what about long web pages?). So based on this, I'd vote for some UI choice.