Mozilla’s thesis is that attackers wouldn’t be willing to go to this next step (actually uninstalling Firefox and replacing it with a different binary) because

Mozilla’s thesis is that attackers wouldn’t be willing to go to this next step (actually uninstalling Firefox and replacing it with a different binary) because antivirus vendors would start treating them like malware. In other words, it’s not a purely technical solution, it’s a political solution, and the success or failure of such a political solution can only be judged by real world results rather than technical possibility.