Hi When I am playing a youtube video in fullscreen in firefox, I sometimes see other programs or videos overlayed on top of the fullscreened video. I have attached an example screenshot. When I google for youtube fullscreen problems, pretty much the only advice I have found is to '''disable or enable hardware acceleration'''. Neither work for me. I am using Ubuntu 16.04, if that helps. https://support.mozilla.org/en-US/questions/1057022 I believe this person had the same problem, but unfortunately for me their problem went away by itself. Mine did not.