MartinSchirr is an AndroidForums member that recently became famous by getting Firefox for Android running on Motorola Milestone. Also known as Fennec, the browser has a guide showing you how to do the same thing over here, but it’s in German, so you might want to use Google Translate. Here’s a video of Fennec on Milestone:
Schirr compiled Fennec for Android and if you’re wondering about the username and password, they’re present on the download page: user – fennec, password android. The test is free, of course and you should at least use Android 2.0 before trying it, possibly because of the OpenGL ES 2.0 support.
Some folks complain about lag or black screens, but you can find out more about fixes and the users’ experiences over here, on the Android Forums. Here’s a demo showing Fennec Addons on Moto Milestone: