I just received a Fire HD 6 generation 4 today. It is a used one, but the price was good. It has been a long time since I owned a Fire, or any Android device as far as that goes. I immediately checked Amazon support and it says the latest FW is 5.1.2.1. I went into System Updates and the device was on 4.5.x.x so I selected to install updates. For some reason it DLed and installed each subsequent update, so it took forever as it incrementally installed each update. When it got to 5.1.2 I thought it was done, but it found and installed 5.3.1.0. Seems kind of strange that their website lists the latest at 5.1.2.1 but apparently there is a 5.3.1.0 available.