fPI will isolate every first-party domain as much as the browser can, including HSTS and other security related properties.

This means that if you visit example.com and then initech.com and both load a tracker from evilshit.org, then the tracker will see too completely fresh browser installs, not even HSTS will be setup if it was used.

Basically, every domain you type into the URL address bar behaves as if it was a unique container.

I didn't know what it was either, until I RTFA:

> "This preference makes the browser associate all data (including cookies, HSTS data, cached images, and more) for any third party domains with the domain in the address bar."

They even included a link you can click on to get more info on it.

Uhm, it's explained in the article.