There was another security release for Flash today as they are addressing some major security issues this week.

I would recommend changing your Flash plugin to a click to play status. Go into about:plugins and click "Always Ask to Activate" for your flash player.

However in your troubleshooting information: " numAcceleratedWindowsMessage: [u'tryNewerDriver', u'9.6'] " suggests to see this: Upgrade your graphics drivers to use hardware acceleration and WebGL

ericoppel said

I have the latest version of Firefox and also the latest flash player. Youtube videos play just fine on internet explorer and google chrome.

Have upgraded graphics driver as suggested but this has made no difference.

Thank you for this, however did you see a difference when using the HTML5 player by disabling Flash?

There was a change in the default player that youtube used this past week for other browsers and this may have affected you.

ericoppel said

I have the latest version of Firefox and also the latest flash player. Youtube videos play just fine on internet explorer and google chrome.

Just figured out how to do this and it worked. Thank you very much for your help!