Clear the cache and the cookies from sites that cause problems.

• "Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites causing problems: Tools > Options > Privacy > Cookies: "Show Cookies"

Thanks that did it.

You're welcome.

I was having similar problems with FoxNew.com videos. Then I read cor-el's response and tried it. The first effort did not help, but a variation did the trick.

I believe it really is a “cookie” problem, ie Too Many Cookies Cached! I found that if I went to: Tools > Options > Privacy it gave me the option of clearing cookies in the middle of the pop-out screen. I cleared ALL cookies, and FoxNews.com appears better, and the videos work again!

Cor-el's response: Link: https://support.mozilla.com/en-US/questions/760682?s=fox+news&as=s

I had the same problem with Fox news videos but found a different culprit. I found it by going through each blocked URL and "allowed" them one at a time while re-blocking those that did not fix the problem. Many anti-virus programs block different URL's including some which are harmless in my opinion. I "allowed" the following URL: PLAYERS.EDGESUITE.NET (this is a Strobe Media Playback for Flash and HTML5) when I "allowed" it a commercial for United healthcare came up in the box to the right of the screen which had stayed white/light gray with the Fox News insignia. If you block the ad to the right of the screen you block the video. You may need to allow it in your anti-virus first. PLAYERS.EDGESUITE.NET is also tied to Facebook if you have the same problem.

 I hope this  helps.