Minefield v1.9.1 on the N800 is the equivalent of Firefox 3.0 Beta 2 on your PC and allows you to use firefox add-ons like adblock plus and noscript.
Add the following repository to the Application Manager to allow you to download and be notified of any updates:-
http://people.mozilla.com/~blassey/ chinook extras
Full details of Minefield repository and alternative download can be found HERE
Minefield deb file is 10MB and once installed takes up about 22MB of space on the internal memory, so ensure you have enough free space.
Running Minefield is about as slow as the built-in browser, but with Minefield you have a totally full internet browsing experience. You need to use the d-pad to open the touch keyboard as the smaller stylus keyboard doesn’t work, personally I use my bluetooth iGO keyboard which works much better.