Issue Description: I’m experiencing a problem with fullscreen mode while using YouTube on Firefox on Windows 11 with a dual-monitor setup. When I enter fullscreen, the video does not cover the entire screen. Instead, I can still see parts of the Windows desktop or background around the edges of the display. This issue only happens on Firefox and only when using fullscreen mode on one of my monitors. I have already checked my display settings, scaling, alignment, and hardware acceleration, but the problem persists. System Details: Windows 11 Dual-monitor setup Browser: Firefox (latest version) Issue: Fullscreen video shows borders and does not fully cover the screen Expected Behavior: The fullscreen video should completely fill the selected monitor without showing any part of the Windows desktop. Please advise on how to resolve this issue.