Hi, thank you for your feedback !
I have though in this functionality too, but there is an add-on already doing it (https://addons.mozilla.org/fr/firefox/addon/youtube-nonstop/). I'm not using it, because I find the code a little heavy and performance eating for this kind of feature. But since I haven't done anything I am not in position of making remarks. If I took some times one day and find a better way I will push it to YoutubeNonStop in order to keep the functionalities separated and do not mix everything :)