It works well.
ver. 56.0.2

love

Ok

It's clear

best quality

خوب وراضی کننده

siiip

simple good

Good

For some videos, the quality goes up to 4K, for example: https://www.youtube.com/watch?v=ys3x43wtuJQ

Console log:
19:11:25.243 sfeerf = Object.assign(sfeerf, JSON.parse('{"hd":true,"quality":"hd1080","log":true}'));
inject.js:120:3
19:11:27.209 YouTube HD:: getAvailableQualityLevels returned empty array
videos:19:15
19:13:36.858 Loading failed for the

Really simple and easy to use addon that does what is described. :)

YouTube often selected 720p or sometimes lower settings, even on a 100mbit connection with a 1440p panel, so I kept having to manually select the quality with every video, but now I don't have to! :)