Right-click in the player area. What player is it using? Flash, HTML5 . . . .

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

FredMc, the video screen is totally black. Nothing happens when you right-click. The Intel driver update says it can't completely install because of an error, but it doesn't say what the error is. This is so frustrating!

and I even did this: Turning off hardware acceleration

   Click the menu button (far right, 3 lines) and choose Options.
   Select the Advanced panel.
   Select the General tab.
   Uncheck Use hardware acceleration when available.

    Click the menu button again and then click Exit (bottom right) .
   Start Firefox normally. 

AND this didn't work

I don't have FB so I called for more help.

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?

that didn't help either.