I've been encountering a problem with YouTube lately in Mozilla Firefox that makes the site inaccessible and displays white space instead of embedded YouTube videos. When I visited the home page, Firefox reported the following "Bad Request" error: "Your browser sent a request that this server could not understand. Size of a request header field exceeds server limit." The error was triggered by the cookie header VISITOR_INFO1_LIVE.
I thought this was YouTube's problem, but when I investigated, I found out that the problem can occur when Firefox's cookies database has become corrupted.
To fix the problem, I opened the folder that contains my Firefox user data, closed the browser and deleted the file cookies.sqlite. When I restarted Firefox, it created a new database to hold them.
The cookies database usually can be found on Windows XP in the folder C:\Documents and Settings
Deleting the file cookies.sqlite wipes out your browser cookies and stored auto-login passwords.