This addon does not download 4K videos. It opens another page on the Internet where you have to pay 19.99$ for this functionality.
That's why the words ''- supports up to 4k videos'' are a lie. Now it does not support 4K if you do not pay.
WARNING! NO 4K
I will find an addon which can do what I want without paying 19.99$ because maybe for you 20 dollars mean 3 cups of coffee, but in my country it is one week of buying food.
IMHO< the author should mention that you have to pay, in the description!
Anyway, I have tried 6 other similar addons, and 2 scripts for Tampermonkey, and none of them did what they promised, they did not download 4K or were not free, or wanted the user to use command line if they need 4K.
I will have to continue using web-sites for downloading YouTube videos, and hopefully Firefox can make it easy for developers to make a functioning 4K downloader.

It is a reliable add on

Doesn't work. Dev is Trash.

One of the best working Download helpers for Videos.

No funciona youtube

Jest OK, używane w youtube.
- add-on pobiera max 720p i trwa to długo (60 KB/s) .
+Jakość jest dobra jak na 720p i nie dodaje żadnych znaków wodnych.

Doesn't work with instagram video story