Amazon Fire TV (Gen 2) is expected to stay mostly in Full HD (1080p) mode at 60 FPS, even on TVs that support 4K UHD format. Fire TV (Gen 2) switches to 4K UHD mode only for 4K UHD content playback.
Fire TV Edition’s hardware decoder supports playback of 4K @ 60 FPS. To play 4K @ 60 FPS, the timing requirement of media pipeline is aggressive and the app may not be able to render 4K frames at 16 msec interval due to thread and process scheduling limitations of the kernel. This may result into frame drops and a sub-par movie experience. To get the best out of the hardware, use Tunnel Mode playback. For details, see Tunnel Mode Playback.
Click here to know more about Crossford