I have all 3 installed and find there's pros and cons with them all.
MicroB has problems with rendering, and plugins are not as readily available. Adblock+ is a pain to set up and sometimes forgets how to do it's job. But it is the faster browser of all 3.

Firefox has a strange and quirky interface, which can be pretty confusing at times. Still under development, so anything could happen. Slow and laggy most of the time, but on a plus, plugins install just like the desktop version and the plugins library is growing.

Opera is way faster than FF in rendering/displaying pages, but still lets a lot of mobile pages through. Nice interface, and a cracking virtual keyboard. I want it for the N900 default. Even though it's smaller it's pretty easy to use.

Someone needs to take the best bits of each one and stitch them together to make THE best mobile browser out there.