Each Firefox download has a unique identifier
Suppose you want to do something anonymously.<p>1. Download installer from Mozilla from your home network - Mozilla now has your home IP and installer ID.<p>2. Transfer it via USB key to a secure, anonymous computer - one not linked to you, on a network not associated with you, such as public WiFi.<p>3. Install Firefox using that installer on said computer. It transmits the installer ID to Mozilla, which matches the one given to your home IP, thereby deanonymizing you.<p>4. Mozilla receives a warrant for this information, or it is hacked, or the organization is infiltrated by a single government or corporate spy.<p>Edit: It gets worse. Suppose a newspaper IT department takes care of providing Firefox and other trusted software installers to their reporters. Now Mozilla can determine who that newspaper helped with IT, such as journalists or sources. Or if you provide trusted software to your friends, Mozilla gets part of your social graph.