There is a serious privacy leak in Mozilla that reveals the URL of the page you are visiting to the web server of the page you visited last. The leak not only occurs for links followed on the page (that wouldn't be particularly serious) but also for URLs entered manually or picked from the bookmarks.
The bug affects Mozilla 1.0, 1.0.1, 1.1 and probably older versions as well. It also affects Mozilla-based browers such as Netscape 7 and Galeon.
The problem is that HTTP requests that are launched from a page's "onunload" handler have the wrong referer (sic): They get the referer of the next page the user is about to visit.
This is bug 145579 from the bugzilla database. It's a couple of months old now so I'm disclosing this vulnerability to hopefully initiate the fixing process.