How? These extensions were not being installed through the normal mechanism. The malicious extension installer will just set the flag that says "this extension has been verified".
> And honestly, I think it is security theater to attempt to defend against attackers on the same or higher privilege level.
I understand that, and Mozilla does too: "By baking the signing requirement into the executable these programs will either have to submit to our review process or take the blatant malware step of replacing or altering Firefox." [1]
[1] https://blog.mozilla.org/addons/2015/04/15/the-case-for-exte...