I searched but couldn't find anything suggesting they're ad-blocking by default -- what they do have, eg https://nakedsecurity.sophos.com/2019/01/30/firefox-makes-ad..., is ability to block trackers. But it's only default enabled in "private" mode.

Could you backup your statement?

Mozilla got a huge sum of money from Google last time I checked (a few months ago), so it would be amazing if they were allowed by Google to enable ad-blocking as a default (unless there was whitelisting that included Google).