[ApK]

This just happens to be the first time that Amazon has had multiple current devices with different major firmware numbers. Previously, it didn't matter if you though the numbers referred to hardware generations or to firmware, you would have been right either way.
Since there ARE in fact now several current platforms at the same time, trying to number them as JS Wolf said would be capricious at best and pointlessly arbitrary and confusing at worst. Since those platforms DO only run one major firmware version each, and there is no reason to think Amazon would suddenly change that practice, the firmware version is the best way to refer to them, especially for a hacking purposes.