I'm using MicroB on Maemo 5 CSSU-Stable on my N900 and that seems to work without problems.
I can understand why it does works this way on firefox on Windows XP and can reproduce it. I think it has something to do with the user-agent that's initiates the request to the webserver. I don't however understand why that would have changed on MicroB on Maemo 4?
Maybe MicroB on Maemo4 never used the user-agent 'Mobile' and Google just updated their browser detection scripts.

But I think we're getting off topic here. Please open a new thread if you wish for other users to help you without sidetracking this one.