Update: It now works exactly how I hoped, and I can change the speed of videos and it works. Changed my rating from 4/5 to 5/5

Amazing add-on. I use it all of the time. Recently it won't let me speed up YouTube videos while I have this add-on enabled which kinda sucks. Before it would just not let me speed it up if this add-on was in use, but now I have to fully disable it from Firefox to speed up the videos.