YouTube videos blank in Firefox

Does anyone know how to get Firefox to show embedding YouTube videos to appear? I can see them in IE but I use Firefox and for some reason–it wasn’t always like this, before I could see them–there is just one big blank space in the post. If the person doesn’t include an html link I have no way to see the video being posted. Is there some setting I can change or what?

Same problem with Firefox and Chrome on Mac. Only Safari 6.0.5 seems to show the videos from this forum nowadays.