Hah. That's precisely the argument I have made when arguing that there should be an opt-out for addon signature verification (needing admin permissions to toggle it if they insist) because you already utterly lost the security game if someone had admin on the machine.

But no, they argue that they must defend against malware with admin permissions injecting addons into the browser. Because that's a fight worth fighting and the perception of the browser's security is somehow more important than user freedom.