Mozilla just released Firefox for Maemo 1.0 RC2, and I post this message with that on whiteg's Nokia N800! It works just fine, except not speedy, and you can install extensions on it!
If you got an Nokia N900, don't forget to try it out. Just go to http://firefox.com/m with your N900 and download the lastest build. Be sure to get the localized version, which include the best search plug-ins set for your region.