Today, Mozilla has itself put out a pre-Alpha build of Fennec that should work at the very least on Droid and the Nexus One. Mozilla is quick to note that this is a pre-Alpha build of the browser, and is only for testing purposes.
To get this build, you can visit this link on your Android device. Or you can point your phone’s browser to: bit.ly/fennec-android.
Whichever way you do it, you’ll need to make sure your settings allow you to install non-Market apps (go to Settings, Applications, and check “Unknown Sources”).