http://spywarcbzke2e6skl7wjwtmmnrkslue24cld6kfcufjhhwxtceogmuad.onion/articles/firefox.html
It sends a lot of different data very often (some of which could uniquely identify you). All the "services" that it provides, such as its default search engines and Pocket, are anti-privacy. The rating isn't higher because at least you can turn off or modify most of it, though it often requires diving deep into about:config.