I'll just drop this comment from the Apps2Fire developer here because it directly affects FireTV hardware while at the same time is software related:
Important: As of Android 6 (marshmallow) Google removed ADB (Android Debug Bridge). ADB is the base function for sideloading. Therefore, Apps2Fire (and similar Apps) will not work anymore. I'm sorry about but I cannot do anything about this. What a pity!
Further to this, there is zero knowledge there and it's like being in a cold white room by yourself due to an unfriendly atmosphere where you feel like you're walking on eggshells not knowing what's allowed and what is.