Mozilla are using a similar model to Microsoft and open sourcing Firefox code under the MPL while distributing the executable under a new Terms of Service.

Firefox (the executable) is no longer considered FOSS since they can revoke your license at anytime. I posted a comment that details my personal view of this change.

TL;DR: they would have only done this if they planned on farming user data. And last month announced that they intend to do just that.