See if there are updates for your graphics drivers https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}

Hi,

Your system details show that you have this add-on installed :

https://addons.mozilla.org/en-US/firefox/addon/youtube-flash-video-player/

One of the reviews says :

"This add-on appears to work by telling YouTube that none of its video formats / file types will play back in its HTML5 player. That **used** to trigger YouTube's Flash player to load. But, YouTube has **completely removed** its Flash player code from its site, so now, all this add-on will do is cause YouTube's HTML5 player to display a message that the browser doesn't recognize any of the video formats available. "

You could disable the add-on and see if that will make a difference .....

Just want to thank you for your help, done quickly to solve my problem. Thanks JF

Unvento said

Just want to thank you for your help, done quickly to solve my problem. Thanks JF

My pleasure - I'm glad everything's back to normal again  !