You can read about it here:

http://bugzilla.mozilla.org/show_bug.cgi?id=147777

but the short description is that a web site can sometimes examine your browser's list of visited sites. This is a problem in the Internet standard so it probably applies to all browsers.

The risk is minimal but you might want to limit how long your browser keeps its history. For Firefox, click Tools > Options > Privacy. Click the plus sign beside History (unless its already a minus sign). Set "Remember visited sites" to 1 day.