A pre-alpha build of Fennec, the mobile version of Firefox, is available for Android devices.
Over the last few months, we've made some great progress on bringing Firefox to Android. Michael Wu, Brad Lassey, Alex Pakhotin and I have been focusing on getting a build ready that's usable by a broader set of people, and we're now ready to get that build out there.
It'll be interesting to see if Gecko can be turned into a worthy mobile competitor to WebKit.
Gecko already runs in a released mobile device. The Nokia N900 uses MicroB as its built in browser and that is based on Gecko. The web browser functionality of MicroB matches, if not exeeds, that of many WebKit based browsers on other mobiles which I think shows that Gecko can be a 'worthy competitor'.