FireCore has released updated Seas0nPass v0.8.2 with firmware stitching which means the install process of old iOS firmware is more easy and simple now.
How to Jailbreak Apple TV 2G iOS 4.2.1 with Seas0nPass
“A new version of Seas0nPass that simplifies the installation of older firmware versions. This process called ‘stitching’ allows Seas0nPass to utilize previously saved firmware signatures to jailbreak and restore an AppleTV version that Apple may no longer be signing.”
You can download Seas0nPass v0.8.2 from source, where you can also find detail tips about Seas0nPass jailbreak tool.