Early build of Firefox for Windows Mobile leaked
The mobile version of Firefox is still in the early alpha phases, (in fact, it's likely that this "leaked" build is just a pre-alpha nightly build of the open source browser) and not only will it not run on some older versions of Windows Mobile (yes, I sadly cling to my Dell Axim X50v running WM2003SE), but it can also cause freezups and crashes on newer devices running Windows Mobile 6. Or in some cases, it will install properly but then it just won't run. The browser also takes up a whopping 22MB of disk space, which might not seem like a lot for a desktop browser, but on a mobile phone Fennec is quite the resource hog.
All told, I wouldn't recommend installing this build unless you want to satisfy your curiousity. It's not really ready for everyday use yet. It's possible that the version Mozilla will officially release to the public soon will be a bit more stable, but I wouldn't expect any miracles.
If you really want to know what Fennec looks like on a Windows Mobile device, you can check out a video from Brandon at PocketNow after the break. And of course, you can always install an Alpha version of Fennec for Windows, Linux or Mac if you want to take the mobile browser for a spin on your desktop. The interface for the desktop version is pretty much identical to the mobile client. The main difference is that the desktop version is fairly stable.