You may only be seeing the window where Firefox should place the video, but the overlay isn't working for some reason. You can try to disable hardware acceleration in Firefox. *Options/Preferences -> General: Performance<br>remove checkmark: [ ] "Use recommended performance settings"<br>remove checkmark: [ ] "Use hardware acceleration when available" *https://support.mozilla.org/en-US/kb/performance-settings Close and restart Firefox after modifying the setting for changes to take effect. *https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems You can check if there is an update for your graphics display driver and check for hardware acceleration related issues. *https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration