Firefox is close to landing some changes in the Firefox development tree that will fix a privacy leak that browsers have been struggling with for some time. The problem is that appearance can be detected by the page showing you links, cluing the page into which of the presented pages youve been to. The biggest threats here are the high-bandwidth techniques, or those that extract lots of information from users browsers quickly. These are particularly worrisome since they enable not only very focused attacks, but also the widespread brute-force attacks (potentially including fingerprinting)”]
Source: https://blog.mozilla.org/security/2010/03/31/plugging-the-css-history-leak/