AFAIK FireOS is ARM only and a brief look on wiki says all the other devices (FireTV, Echo, Echo Dot, etc) are all ARM.

Either way, I'd imagine you would want to wipe it completely. Can't wait for the iFixit teardown.

FireOS is just Android with a UI skin and some Google services ripped out. I'd be surprised if an x86 fork wasn't always maintained, if for nothing else than ease of debugging!

FireOS is based on Android which definitely runs on x86