Important context!
They had to change this because newer laws like the CCPA classify some ways of transferring/processing data as a "sale", even if no money is exchanged.
See: this Firefox FAQ where they say:
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 broad and evolving. As an example, the California Consumer Privacy Act (CCPA) defines “sale” as the “selling, renting, releasing, disclosing, disseminating, making available, transferring, or otherwise communicating orally, in writing, or by electronic or other means, a consumer’s personal information by [a] business to another business or a third party” in exchange for “monetary” or “other valuable consideration.”
Similar privacy laws exist in other US states, including in Virginia and Colorado. And that’s a good thing — Mozilla has long been a supporter of data privacy laws that empower people — but the competing interpretations of do-not-sell requirements does leave many businesses uncertain about their exact obligations and whether or not they’re considered to be “selling data.”
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. We set all of this out in our privacy notice. Whenever we share data with our partners, we put a lot of work into making sure that the data that we share is stripped of potentially identifying information, or shared only in the aggregate, or is put through our privacy preserving technologies (like OHTTP).
We’re continuing to make sure that Firefox provides you with sensible default settings that you can review during onboarding or adjust at any time.