In theory websites can use your browsers cache to determine if and when you visited another website. To do this a website would simply have to know which images, or other files, are used on the website that is probed. So, if image test.jpg is loaded from the cache instead of the second website it is safe to assume that the user has been visiting the first website as well.
Safe Cache and Safe History are two Firefox extensions that are able to prevent this methods of finding out.
According to Martin, who posted this info on gHacks, “Both Firefox add-ons are recommended to increase security.”
Both add-ons are by Collin Jackson.
