http://spywarcbzke2e6skl7wjwtmmnrkslue24cld6kfcufjhhwxtceogmuad.onion/articles/firefox.html
When you then open a new tab, Firefox will sometimes make requests to the sites in there, including some of their trackers. I haven't determined how it works yet. Sometimes it doesn't make the requests at all; other times you end up with hundreds of images, scripts, trackers, etc. loaded simply because you opened a new tab (without visiting any website explicitly).