I suspect MicroB beats fennec on start-ups because as I understand it, aspects of the MicroB browser are preloaded into memory when the N900 boots. So microb is always partially open. (Keep in mind Conversations uses modified browser windows to open up each conversation too, so the part of microb that's always open is multi-purpose.)

Honestly though, if only Fennec used the stock status/menu bar instead of their own at the top, I would switch to whatever the latest nightly build is. I would /strongly prefer/ to also have the swirl-to-zoom gesture and the mouse cursor selection, but even with the lack of those I'd be willing to convert.

(Not so much to Opera as my main browser, but that's mainly because I don't like non-desktop pages and at some level I still care about having flash, though the constantly worsenning out-dated-ness of our flash player makes that less and less relevant. But major respect to Opera for continued support. I do use it on my N900 occasionally, even if I'm unwilling to use it as my main browser.)