This is yet again a nothing burger. They had to change that definition because in California "selling" means "[...] releasing, disclosing, disseminating, making available, transferring [...]" in exchange for “monetary” or *“other valuable consideration.”*. You can see user collected data publicly, and use said data to improve Firefox the browser, that would be argued that is a valuable consideration.
The user collected data are things like about:studies, the new tab page, Firefox Addons, and anything listed here https://www.mozilla.org/en-US/privacy/firefox/#health-report
This is literally language that had to change to describe Mozilla's actions in line with California's law.