YouTube/Video Problems: What might be happening? (Details follow.)

I’ve been having problems with YouTube and other embedded videos (in other websites) for a few days. What happens is that I will open up a video and it starts to play okay. Then it stalls, and all I hear is the voice (the action freeze-frames). Then, even the voice stops. When I try to close the window, it refuses to shut. I also cannot open any other windows either. What I end up having to do to “unstick” the video is to close all the windows so that the video shuts down.

I’ve never had this problem before. Do you know what might be causing this? (It is also happening with other videos that I find on other sites….seems to be an overall problem.) I use Firefox and have Windows 7.

Thank you.