Why isn't YouTube working for me?

I have a Macintosh OS X 10.5.6. I always work off of the Safari browser, and just yesterday night YouTube stopped working for the browser. It says, "Your browser sent a request that this server could not understand. Size of a request header field exceeds server limit." So I downloaded the Firefox browser to try YouTube and it's works. So what's the problem with Safari? By the way, all other websites work for Safari.