Macbook won't play videos now

Hey everyone,

I'm a fairly new mac user, and have loved my macbook so far. I

don't know what settings I could have changed, but for some reason, my

macbook won't play video's. I can go to youtube, open a video, but it

won't play for some reason. This is something that has just started. I'm using Firefox as my browserAny

suggestions?

Thanks,

Dougal