http://spywarcbzke2e6skl7wjwtmmnrkslue24cld6kfcufjhhwxtceogmuad.onion/articles/firefox.html
Allegedly used to protect you from "phishing" websites, but in the end, it makes a bunch of requests to Google every 30 minutes (according to Mozilla), including a POST request with your Firefox version and a unique, persistent, hidden cookie.