Colcaveat

I have Windows XP Home Edition Service Pack 2 and Ver. 1.29.295. When I run CCleaner, I get the following message: "You must close Firefox/Mozilla to allow the Internet Cache to be cleaned. Otherwise this part will be skipped". But Firefox/Mozilla is already closed, yet, on cleaning, the cache has skipped. Obviously, I am doing something wrong here, and any asistance would be appreciated.