Firefox has a flaw that could allow attackers to see where you've been on the Web. The attackers can create a bitmap file that lets them pull information from the memory of the browser, including parts of other Web sites, users' favorites, and history.

The article specifies Foxfire 2.0.0.11, so be sure to update to Firefox 2.0.0.12, which has corrected the flaw.

There are other tricks to do this, one nice example is to have a:link and a:visited styles with different widths. You can then check the widths using Javascript, and it will tell you whether the visitor has been to that page before. Cross-browser compatible too.