After seeing the leaked version of Firefox Mobile hit the web, here’s the real pre-alpha release of Fennec, which should be a tad more stable than the leaked one. You can download the cab installer from right here, but keep in mind that this is release is targeted specially to HTC Touch Pro users, so it might run with issues on devices with non-VGA screens.
Also, it turns out that text entry without a physical keyboard is a no-go, so you should really be using a Touch Pro. The pre-alpha is used as a method to get early adopters to post their feedback and bug reports, in order to clean up the code of the software. In case you own a HTC Touch Pro, you should navigate to the cab installer (link here) with your browser and download it, starting the installation process afterwards.
It’s worth mentioning that the browser doesn’t have the update mechanism activated, that it has the plug-in support disabled and, as mentioned before, it doesn’t support soft keyboards.