I saw that many people are complaining, for me it works fine.

What I like about this app is that it doesn't try to overwhelm you with features. It controls or over rides all the Youtube settings that I want to adjust and stay that way. I particularly like this app for my laptop as I usually use it while on mobile data, so I set it to 480p and it uses significantly less data than 1080p and I can't notice the difference. You can also prevent it from playing the next video or set the video to auto pause, all these features save my data usage and once the extension is installed and your preferences selected it will never change back, "Excellent"!! (Monty Burns).

Its great but has a bug in it like a month long, so maybe since last update,even reinstalled firefox to avoid the mistake on my end!The bug is that when you open youtube that any specific page will be continously loaded so that its impossible to search,play a video or do anything ! So at the moment i disabled this Add-On...hope it will be fixed soon!
Five stars cause it was great and im sure it will be working again when the bug gets fixed !