[Osiris]

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. In the case of Safe Cache the user has the option to select one of the following privacy levels: segmented cache, cache originating site only, or never cache. For Safe History it is: segmented by origin, don’t mark links visited in offsite frames, or never mark links visited.
A test website has been created that uses a method to determine your online bank. This probably does only work for US online banks and only the user visited a website of a bank and did not delete the cache in Firefox afterwards. Both Firefox add-ons are recommended to increase security.


Read More:
Proof of concept
Safe Cache
Safe History