Does Firefox display the plugin notification icon in the address bar? It looks like a small gray Lego block. You can click the icon to confirm that the page has permission to run Silverlight, or use the buttons on the notification panel to give the page permission.

If the page has permission and there are no error messages displayed, hmm, could you check for any issues in the Browser Console? You can open that using Ctrl+Shift+j (or menu > Developer > Browser Console). Then click the "Clear" button the console toolbar to remove the clutter switch over to Firefox to reload the page and start the player, and switch back to the console to check for any error messages.

You may find some style warnings, those probably aren't the issue. Anything else that looks interesting?

If you use extensions ("3-bar" menu button/Tools > Add-ons > Extensions) that can block content (e.g. Adblock Plus, NoScript, Flash Block, Ghostery) then make sure that such extensions do not block content. There is security software that can interfere with loading content, so check that as well.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thank you jscher2000. The Browser Console helped. Apparently I cannot use the strict setting for the "Tracking Protection in Private Windows" (which I have enabled for all other windows as well in the Firefox config). I have to use the basic setting. :-( At least now I know.