It’s in the actual Terms of Use that’s linked in the article: https://www.mozilla.org/en-US/about/legal/terms/firefox/#you-give-mozilla-certain-rights-and-permissions

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.

What? This sounds way too broad, as if everything we do can be used by Mozilla to do whatever they want.