It began yesterday, after I updated Firefox to 13.0.1. I've restarted my computer and I've run Firefox in safe mode. Nothing makes a dent.
When I try to play a video, rather than it playing after I click it, I just see the rectangle of the video with the "play" icon in the middle. If I click it, it says "An error occurred. Please try again later."
I just tried YouTube in Chrome and it goes perfectly.