I'm on nightlies, and with the latest 20100128 runtime it wouldn't start for me, so I uninstalled Firefox (which had the orange Firefox logo) and went to reinstall Firefox but could only find Fennec 20100128 (with the blue Fennec logo) so I installed that and now it's working again but I can't say I'm that impressed by the speed. Firefox/Fennec RC1 is in a Mozilla hosted repository, and since it's not actually released yet (it's marked as "Beta" on www.mozilla.com/mobile/) if you didn't want to help with the testing you shouldn't have installed it!