The solution by CarstenDutch will work but some people like myself dont know anything about SSH and stuff like that so

Here are some tips to get it work

1. Uninstall 1.0.2 version if you installed from repository

Restart!

2. Get access to .mozilla/micorb/extensions ( I've used Cute Explorer, Show Hidden option enabled) and see that only one folder is there starting with {ce402fde.... ( Ovi Suite automatic login plugin) If you dont have other plugins then delete everything except that folder

3. Follow instructions HERE

NOTE: When you click "Click here to download XPI extension" on N900 it will install addon automatically
Now the tricky part, even though MicroB shows icon to restart itself actually it doesn't do it so you have to do it manually

4. Without closing the browser go to x terminal and type

5. Open MicroB and follow the rest of instructions on the page from 3rd step

6. When install is done and filter is loaded you can just check using a webpage or via CuteExplorer, I've circled the folder that should be there

Cheers
Let me know if its not working