Ah, yes, certificate patrol[0]. However, your argument, I think, is not valid. Sure, it's technically possible for him to know if any cert changes, but in reality very few people are going to install the extension and those that do might not even notice the message because it notifies the user so frequently (fully desensitizing them I imagine).

I don't think that his choice not to install an extension invalidates his argument.

[0]: https://addons.mozilla.org/en-us/firefox/addon/certificate-p...