You can try to disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

You need to close and restart Firefox after toggling this setting.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes
• https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

Try to "disable the hardware acceleration" in the Flash Player.

• How do I disable or enable hardware acceleration?
  https://forums.adobe.com/thread/891337
• https://helpx.adobe.com/flash-player/kb/video-playback-issues.html

Flash "Display settings" window:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help01.html

None of that seemed to help. I disabled Hardware Acceleration for both Firefox and Flash Player. I still have the same issue.

Hi Chuckk. 1. Go to Youtube. 2. Once the site has loaded, click on the small lock icon in the location bar. 3. Set the "Enter Fullscreen: " permission to "Always Ask" or "Allow".

(Don't forget to re-enable things like hardware acceleration if my solution worked.)

kenkon said

Hi Chuckk. 1. Go to Youtube. 2. Once the site has loaded, click on the small lock icon in the location bar. 3. Set the "Enter Fullscreen: " permission to "Always Ask" or "Allow". (Don't forget to re-enable things like hardware acceleration if my solution worked.)

Jeez O' Pete! That did it! Thank you so much. I have no idea how that got switched off. I didn't even know it was there, and I've been using Firefox for over a year now with no problems.

Thank you again for solving this very annoying issue for me!