Another user reported the same problem. But when I checked it out, the video played normally. If you have problems with current Shockwave Flash plugin versions then check this: * see if there are updates for your graphics drive drivers '''https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration''' * disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later) '''https://forums.adobe.com/message/4468493#TemporaryWorkaround''' * disable hardware acceleration in the Flash plugin '''https://forums.adobe.com/thread/891337''' See also: * '''http://kb.mozillazine.org/Flash#Troubleshooting''' ------------------ Start '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Firefox in Safe Mode]''' {web Link} by holding down the '''<Shift><br> ''(Mac Options)'' ''' key, and then starting Firefox. Is the problem still there? ------------------- Type '''about:support''' in the address bar and press '''Enter.''' Under the main banner, press the button; '''Copy Text To Clipboard.'''. Then in the reply box at the bottom of this page, do a right click in the box and select '''Paste.''' This will show us your system details. '''No Personal Information Is Collected.'''

Could be that you have enabled two pane 3D view for the YouTube media player, so check this via the settings (gear icon).