Definitely MicroB, the idea of firefox is great but even with flash enabled I just find it pales compared to the proprietary browser. I think the n900 for a lot of non-tech/semi-tech people (like myself) is all about full fat web on a little machine and while firefox is ok in theory and the mutt's nuts in terms of concept, for me MicroB seems totally intuitive and a winner after a month and a half of use.

To be fair though either option is still miles ahead of anything else out there as far as I'm concerned.