Specifically it's this section:
You give Mozilla all rights necessary to operate Firefox, including processing data as we describe in the Firefox Privacy Notice, as well as acting on your behalf to help you navigate the internet. 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.
They updated their blog with:
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.