I am Using the new HTML 5 Player when watching Videos on Youtube. It works just fine when media.mediasource.enabled is set to "false" but then I cannot get the full experience because the lack of features! If I enable it, NO Video is going to load, meaning I cannot watch any Video on Youtube! As a workaround I installed the Plugin "Youtube Control Center" (missing in your List below). But it would be nice if I would NOT have to use this... Even better would be if Firefox would Support WebM. But this does not seem to be the case since YT uses exactly this Format with its new Player... (And if I enable this media.mediasource.enabled (set it "true") NOTHING works anymore.)