Do you have this problem primarily on Google sites? I'm wondering whether the Google Disconnect extension might be blocking something important that YouTube or other Google sites expect to receive.

Could you test in Firefox's Safe Mode? That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

Hi, I followed your instructions and I'm able to view video. Do I need to stay in Safe Mode or is there a permanent fix?

Thanks!

Safe Mode isn't practical for permanent use, but you can try making some of the changes in regular mode.

(1) Disable hardware graphics acceleration in Firefox (this is disabled in Safe Mode)

Un-check the box here and restart:

Firefox menu > Preferences > Advanced > General > "Use hardware acceleration when available"

(2) Check for and disable any nonessential or unrecognized extensions on the following tabs (these would all have been disabled in Safe Mode):

Tools menu > Add-ons > Extensions category

(3) If you use any recorders/downloaders that interact with Flash media, and you do not want to disable them, make sure they are as up-to-date as possible