While Firefox Mobile for Windows Mobile has not been released yet, one can get an early taste of the browser in action by running the desktop emulator version, which can be downloaded here.
GSMDome has done just that, and has recorded this video of the application in action.
As can be seem from the video, the rendering is pretty good. However in my experience, while double tap zooming works OK, there seems no way to do variable zooming as in Opera Mobile.
Read more about the release and download the emulator here.