FireCore has successfully jailbroke Apple TV 2G running iOS 4.2.1 with a new jailbreak tool called Seas0nPass, for now it’s only available for Mac users.

Download Seas0nPass jailbreak for AppleTV
Download iOS 4.2.1 for Apple TV 2G
Download iTunes 10.1.1

STEP 1: Unzip the ‘Seas0nPass.zip’ file and launch Seas0nPass (Download at the end).

STEP 2: If your AppleTV is not yet jailbroken, select ‘Create IPSW’. (if already jailbroken, and you just need to do a ‘tethered boot’ scroll down).

STEP 3: The latest AppleTV software will be downloaded and used to create a custom ‘jailbroken’ software file.

STEP 4: When prompted, connect your AppleTV using a micro-USB cable (leave power disconnected), and hold both the ‘MENU’ and ‘PLAY/PAUSE’ buttons for 7 seven seconds.

STEP 5: iTunes will open automatically and start the restore process.

STEP 6: iTunes will confirm the restore when complete – your AppleTV is now jailbroken. Please proceed to the ‘tethered boot’ section below.

How to use Seas0nPass for a ‘tethered boot’:

Note: The AppleTV MUST be disconnected from the computer (for at least a brief moment) after restoring in iTunes for the tethered boot process below to work properly.

STEP 1: Open Seas0nPass and select the ‘Boot Tethered’ option.

STEP 2: When prompted, connect the AppleTV to your computer via USB. Wait for the rapid flashing light then connect the power. Once both are connected hold both the ‘MENU’ and ‘PLAY/PAUSE’ buttons for 7 seven seconds.

STEP 3: Seas0n Pass will complete the tethered boot automatically. Once complete, remove the USB cable and connect the HDMI cable.