That's too much less info.
? What is the current version of the browser?
? So have you tried to update the version of the Firefox?
? Does this happens in other browser too?
? Have you updated graphics drivers?
And lastly,
? Is there some genuine reason to stuck with XP?
Also, I am guessing since you have an LCD monitor, your PC is capable enough to handle Ubnutu or Windows 7? If not, kindly ignore this point.