From /.

“The Mozilla Team has quietly enabled a new feature in Firefox that parses ‘ping’ attributes to anchor tags in HTML. Now links can have a ‘ping’ attribute that contains a list of servers to notify when you click on a link. Although link tracking has been done using redirects and Javascript, this new “feature” allows notification of an unlimited and uncontrollable number of servers for every click, and it is not noticeable without examining the source code for a link before clicking it.”

Why should browsers bother about redirects?