SmartYouTubeTV

SmartYouTubeTV: An YouTube client for set-top boxes and Android-based TVs:

Features:

4K support.
Designed for TV screens.
Any remote controller support.
Login support.
Doesn't depend on Google Services.
Fully localized.
Multilingual search keyboard.
Automatic update mechanism.
No root required!

NOTE: SmartYouTubeTV provides internal login mechanism which allows you to keep content up to date in a very convenient way.

Screenshots:

Open screenshots

Install SmartYouTubeTV:

Easy installation in less than 5 minutes with only the FireTV

Install SmartYouTubeTV (only FireTV needed)

Standard installation via ADB

If you don't know how to sideload/install apps via ADB, read a tutorial (e.g. this one)
Download latest SmartYouTubeTV APK and sideload/install with adb:
adb install -r SmartYouTubeTV_Orig.apk
Enjoy :)

How to change system language with only the MiTV3

How-to-enable-English-on-Android-5.1

Starting screen:

The device/resolution options at start launches different playing methods (as per yuliskov#15 (comment))

1080 Main - WebView
1080 Alt - XWalk
4K Main - Exoplayer + WebView
4K Alt - Exoplayer + XWalk

Also important, it's possible to select max resolution and codec (ie 1080/AVC1) via the start screen that will be applied on video loading (Codec option).

The Remember tick box acts as a configuration saving for next app launches.