TLDR Firefox has been selling your data all along in exchange for ad money, but now state laws with more teeth forced them to come clean about this behavior.
Could you quote where it says that?
> The reason we’ve stepped away from making blanket claims that “We never sell your data” is because in some places, the LEGAL definition of “sale of data” [is the transfer] of a consumer’s personal information [from one business to another in exchange for something of value].
> [...]
> In order to make Firefox commercially viable, there are a number of places where we collect and share some data with our partners, including our optional ads on New Tab and providing sponsored suggestions in the search bar.
This is them saying "it's not that we've suddenly become more evil... we've been doing this for a while... we gotta make money somehow, and advertising and sharing your data is how we do that, but now state privacy laws make us have to be clearer about it".
Firefox gets almost all of its money from Google Search sponsorships and other ads. (https://www.investopedia.com/articles/investing/041315/how-m...). It's not really that different from any other adtech company. It's just one degree away, but most of that sweet user data still flows to Google in the end. Sure, they might obscure some of the PII... but so did FLoC, Google's controversial attempt to keep tracking users after third-party cookies.
Firefox is just a privacy laundering operation for Google and some smaller advertisers. Then Mozilla uses most of that money on unrelated marketing and virtue signaling, pretending like they're some sort of privacy / civil rights champion, when in reality they're not really very different from any other ad-based browser maker — except that they're horribly inefficient at using their millions. All that money and Firefox has still fallen way behind, all while Mozilla keeps pretending they're some sort of enlightened think tank. Nobody actually pays attention to any of their think-tank related work or their other services. Either as a browser maker or a privacy-oriented nonprofit, they're completely ineffective.
If Google stops funding them, they'd shut down overnight, losing 90% of their revenue. And maybe that's a good thing... it's time for a more capable org to take the reins. Mozilla has been a terrible steward, and Firefox went from the thing that saved the internet (from a Microsoft IE monopoly and the super-bloated app suite that Netscape Communicator / Mozilla Suite became) to then crumbling under the poor leadership of its lost decade.