As usual, this WILL void your warranty and I am NOT responsible for anything you do with this. Using this tool won't brick your Fire TV, but doing stupid things afterwards might. I will stress here that flashing something bad to boot is a guaranteed way to brick because if the kernel can't boot you have no way to get back to recovery. For testing kernels, it's best to flash recovery to the boot partition and the new kernel to test to the recovery partition.
- Downgrade: You must first downgrade to 126.96.36.199. If you already have CWM installed, you can skip this.
- adb push aftv-unlock /data/local/tmp/
- adb shell
- chmod 755 /data/local/tmp/aftv-unlock
- /data/local/tmp/aftv-unlock unlock
Afterwards you should see this message:
Attempting to unlock... All done!