Videos played on Youtube or Netflix are partially transparent and show the window(s) or wallpaper on the desktop behind.

I'm attaching a screenshot of playback of a full screen Youtube video. Behind this, there was another Firefox window with an (old) Youtube logo in the top right quadrant. This logo in the window behind shows through the full screen video playing in front of it. You can see it in the attached screenshot right above the tip of the flare. This happens only with Firefox and Firefox Developer Edition and not with Chrome.