YouTube stopped working on Firefox for me yesterday. How to fix?

I was watching videos yesterday, and all the sudden, I clicked on another one, and it stopped working. Specifically, I mean that the full page loads on YouTube, but instead of the normal video screen with controls (play button, scroll bar for length of video, etc), I just get a black box where the video should be.

I tried Internet Explorer, and it’s working fine there. But I use Firefox, and want YouTube working on my Firefox. It had worked fine for a long time till yesterday. Any idea how I can fix it?