I'm very happy with this addon it gives me what I need on youtube (slow/fast speed, default *4K, HD).
I wish this kind of add-on can be added on android ;) (mmh maybe it can)

*Sometimes, it doesn't shows the 4K as default (why?).

This addon is very helpfull but it has some bugs - it spams many same elements (style and script - including usermade) into head of document. It can make browser slower as well as it takes more memory.

I love all the little adjustments the extension allows, as well as the customisation, which is a very nice aspect that YouTube itself doesn't seem to care much for. However, there seems to be a conflict with the Remove Youtube Suggestions extension which causes it to not work, so I'm forced to keep it turned off :/
I hope the conflict is fixed soon, so I can go back to using this.

Me encanta, muy útil

Amazing! Recently upgraded to Firefox and there seems to be a picture in picture mode (Float the video on top)!!!!!!!!1

Best add-on but preference for a default download path would be a great improvement in functionality.