Strange "ghost images" / transparency effect in full-screen videos
My config is Firefox 64.0, Ubuntu 18.04.1. Experienced the same problem. After searching and reading, finally was able to fix it.
Changed layers.acceleration.force-enabled
option to true in the about:config
Edit: You may also try to create mozilla.widget.use-argb-visuals
as Boolean and set it to false in about:config
Source: Firefox Bug 1516224 Semi-Transparent YouTube video playback