[shakna]

> If you use a static URL, you'll know if you have a new session or not, but that doesn't tell you what the tracking ID was.

As Mozilla have said:

> "In the case of Firefox’s image cache, a tracker can create a supercookie by “encoding” an identifier for the user in a cached image on one website, and then “retrieving” that identifier on a different website by embedding the same image."

The identifier is encoded into the image itself on a fresh fetch of the static URL, which can then be extracted by JS (which can access pixel data, and their RGBA channel values).

When a cache-hit is detected, you know you have an identifier that correlates to user history.