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
In about:config setting gfx.xrender.enabled to true fixed it for me, with no rendering issues and no loss of performance.