Video is generated with alpha color. We fixed that by disabling ARGB visual for toplevel window in X11 (see Bug 1516224) but we can't do that on Wayland.
It affects fullscreen playback on Wayland but it's also reproductible on X11 when mozilla.widget.use-argb-visuals is set and ARGB visual is used for toplevel window.
- set a desktop background with some high contrast colors / shapes
- set mozilla.widget.use-argb-visuals to true or run Firefox on Wayland as
- play video at
- try fullscreen (wayland) or normal playback (x11)
- background shines through video when the video is running. When it's stopped it's ok.