The first "user-enhancing" in-browser ads ("sponsored tiles") were introduced in 2014 https://twitter.com/dherman76/status/433320156496789504
Pocket imposition was 2015 https://bugzilla.mozilla.org/show_bug.cgi?id=1172126
Mr Robot stunt was 2017 https://www.cnet.com/tech/services-and-software/mozilla-back...
Cliqz tracking was 2017 https://www.zdnet.com/article/firefox-tests-cliqz-engine-whi...
booking.com ads was 2018 https://venturebeat.com/2018/12/31/mozilla-ad-on-firefoxs-ne...
It's easy to look at the actions of the last few years as if they are the exception, but this has been going on for a long time and Mozilla was given a slide for "reasons"