Nothing to do with Firecore.
Everyone’s waiting for DevTeam to provide an untethered jailbreak for AppleTV on iOS 4.2.1
In the meantime, you could try to do a tethered jailbreak, but you really have to know what you are doing and have a Mac running Snow Leopard and a Windows virtual machine close to the AppleTV box.
Not for the fainthearted and it took me 2 weeks to get it right - needed iReb to kill Error 1600, use Windows to restore the firmware and had to reinstall stock firmware first if I got Error 21.
But I now have AppleTV with aTV Flash and AirPlay all running beautifully.
Info here: http://thetechjournal.com/electronics/how-to-jailbreak-apple-tv2-on-ios-4-2-with-pwnagetool.xhtml