For those who don't want to read a GitHub diff message - and who can blame you? - the removed FAQ text:

    Firefox is independent and a part of the not-for-profit Mozilla, which fights for your online rights, keeps corporate powers in check and makes the internet accessible to everyone, everywhere. We believe the internet is for people, not profit. **Unlike other companies, we don’t sell access to your data.** You’re in control over who sees your search and browsing history. All that and exceptional performance too.

And, later...

        {  
            "@type": "Question",  
            "name": "Does Firefox sell your personal data?",  

"acceptedAnswer": {
"@type": "Answer",
"text": "Nope. Never have, never will. And we protect you from many of the advertisers who do. Firefox products are designed to protect your privacy. That’s a promise. "
}
}

Firefox claims (both in the above link and in a new privacy page) that this is all a misunderstanding:

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).

However, people have also noted a new addition to its Terms of Service, added some time between February 9th and February 26th:

When you upload or input information through Firefox, you hereby grant us a nonexclusive, royalty-free, worldwide license to use that information to help you navigate, experience, and interact with online content as you indicate with your use of Firefox.

Again, Firefox has clarified:

UPDATE: We’ve seen a little confusion about the language regarding licenses, so we want to clear that up. We need a license to allow us to make some of the basic functionality of Firefox possible. Without it, we couldn’t use information typed into Firefox, for example. It does NOT give us ownership of your data or a right to use it for anything other than what is described in the Privacy Notice.

But people - like myself - are worried that in a legal sense, the vaguely comforting blog posts don't matter nearly as much as the TOS and privacy policy changes, and that what these caveats amount to is "We won't sell your data...unless we really want to." I highly recommend reading through the changes if you use Firefox and making your own decision about whether or not you believe them.