@p That's their explanation, but it literally says: "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

This is not optional.

https://www.mozilla.org/en-US/about/legal/terms/firefox/