Mozilla released a pre-Alpha build of Fennec that should work at the very least on Droid and the Nexus One. 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. And, there’s a QR code on Mozilla’s Vladimir Vuki?evi? blog post if you want to scan it, and download it that way. Whichever way you do it, you may need to enable installation of non-Market applications by going to Settings, Applications, and checking "Unknown Sources".
More info: Fennec on Android