OK, not really, but Web trackers are out there on the Internet. This is a test page for Firefox's built-in Tracking Protection.
If you're running Firefox 42 or later and viewing this page in a Private Window, you should see a shield icon in the URL bar and the first two (all three on Firefox 49 or later) of these should be green:
- a simulated third-party tracker was ...
- a simulated first-party tracker was ...
- the DNT signal was ...