Went to view some movie trailers on the following two Apple websites:
Independent - Movie Trailers - iTunes
iTunes Movie Trailers
Guess what? I couldn't because the Quicktime plugins had been disabled in Firefox for Mac OS during my upgrade to El Capitan. After 10 minutes of research discovered that this was deliberate by Apple for the following reason:
If you visit a webpage that requires the legacy QuickTime 7 web plug-in, your web browser might display a message that the plug-in is missing or not supported. This plug-in is no longer enabled in OS X El Capitan, because websites increasingly use the HTML5 web standard to deliver a better video-playback experience across a wide range of browsers and devices.
So Apple hasn't even updated their own websites to comply with their own standards - very poor, get your act together Apple.
The fix for this is on the same page as the above quote: If your web browser says that it‘s missing the QuickTime plug-in
