When watching videos in Firefox, I can see the outlines of the windows I have open in the background regardless of whether the video is fullscreen or not. If the video is paused, the background windows are not visible. This happens at least on Youtube, Twitch and Netflix. The videos look normal in Chrome, so this could just be an issue with Firefox, in which case I will go to Mozilla for help.
I couldn't find anything related to the issue as the search results were mostly web development related. I looked into about:config in hopes of finding a related setting, but didn't find anything. I also tried with a new profile to rule out any addons and config changes as the cause. Disabling HW acceleration didn't help either.
Any ideas what could be the cause? Could this be an issue with the X Server? I have Ryzen 2600X and RTX 2080.