https://firefox-source-docs.mozilla.org/toolkit/components/t...
Or maybe it's because you can toss some money at Mozilla and they'll silently install a browser extension on behalf of an advertising company for a shitty TV show about a l33t haxx0r d00d?
By the way, the bugzilla report about that particular incident was locked, then made employee-only, then made public again...then restricted beyond employee-only...by a project manager...who used to work for advertising companies before she came over to Mozilla.
Or how about the fact that flipping on some of the anti-tracking features include munging the timezone, which means times in almost any website are wrong - which seems to be a poison pill to keep people from turning it on?
Edit with details regarding the extension controversy: https://www.theverge.com/2017/12/16/16784628/mozilla-mr-robo...