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.

Doesn't work. Dev is Trash.

No funciona youtube

I would give zero if possible.
This extension literally does nothing. Complete piece of dung.

Ad riddled malware

NOT WORKING.

Doesn't work, shows only plenty of ad

was not able to download anything above 640p

Unable to download some videos and recomends to pay for some desktop application claiming it is impossible doing so in the add-on and recomending to pay for a desktop application, even though tools like hls-downloader are able to do it in an add-on.

A hidden money maker machine.
It can download fewer file types than other add-ons and then invite you to buy a program from 13.99 to 24.99 dollars.
This should be on the description of this extension.

Garbage. All this addon does is redirect you to a page to download the pro version. Dont waste your time