They opted firefox users into a data collection scheme they call PPA which works kind of like FLOC and uses the browser to gather information about what you do online, then they sell that data to advertisers by first sending it to yet another a third party who will assemble that data into reports for the advertisers. Then they basically said firefox users were too stupid to be trusted to opt-in, and it would be too hard to explain to such dumb users how selling their data was a good thing, so Mozilla had no choice but to force it on everyone by default without telling them about it. (https://web.archive.org/web/20240715112635/https://mastodon....)

Naturally not everyone was happy about it:

https://noyb.eu/en/firefox-tracks-you-privacy-preserving-fea...