I am experiencing the same issue as of today on all videos. I am using Firefox Quantum 66.0.5, Violent Monkey 2.10.5, and ViewTube 2019.5.14.
I just discovered a movie that this DOES work on: https://www.youtube.com/watch?v=4vZbWXPp46s. So the situation appears to be related to movie resolution. (I read someone else's comment and thought they meant movie length.)
Viewtube not working.
I just installed ViewTube 2019.5.1 from the Chrome store and it appears on top of the Youtube video for half a second and then it dissapears.