It seems Members First might be a new thing from Youtube. My usual Members Only blocker would not stop them, nor would any other Members Only blocker I tried. So, I searched for "Members First" and found this. This is the first add-on so far that I have tried to stop these Members First videos that are just as annoying as the usual Members Only.
While this does work it has an odd behaviour on the right hand side of a playing video. A members first video there keeps appearing and disappearing when my mouse moves. My other members only add-on has been updated and does not have this problem. So, I am disabling this and going back to an alternative.