If they were only selling data in such an 'innocent' way, couldn't they clearly state that, in addition to whatever legalese they're required to provide?
That's literally exactly what they do. This is why you should consider reading beyond headlines from time to time.
> You give Mozilla the rights necessary to operate Firefox. This includes processing your data as we describe in the Firefox Privacy Notice. It also includes a nonexclusive, royalty-free, worldwide license for the purpose of doing as you request with the content you input in Firefox. This does not give Mozilla any ownership in that content.
> (from the attached FAQ) Mozilla doesn’t sell data about you (in the way that most people think about “selling data”), and we don’t buy data about you. Since we strive for transparency, and the LEGAL definition of “sale of data” is extremely broad in some places, we’ve had to step back from making the definitive statements you know and love. We still put a lot of work into making sure that the data that we share with our partners (which we need to do to make Firefox commercially viable) is stripped of any identifying information, or shared only in the aggregate, or is put through our privacy preserving technologies (like OHTTP).