I have been playing with this game for quite a while and it seems I'm a bit addicted.
This game is worth playing. You should try it.
Below is a screenshot.
This tutorial is made for you guys with a Kindle Fire HD 7" (2012) running the 7.5.1 software update. This can be done without using a Fastboot Cable. The important thing here is that you will downgrade the OS to 7.2.3.
I know there are a lot of tutorials out there on how to install custom ROMs on a Kindle Fire specially on XDA Developers' Kindle Fire Android Development Forum but this one is specifically for a Kindle with 7.5.1 firmware. Basically, this is a summary of tutorials on the internet. I have included the links to Youtube tutorials as well which are very awesome. They made it possible for me to root and flash a custom room.
I don't know if you will follow this tutorial but trust me on this one.
If you have any questions, please feel free to post it as a comment down below.