That browser extension exists, or at least it used to. Of course, you run aground of the fact that nobody signs their zones (other than European domains whose registrars sign them automatically, which is, when you think about it, just theater).
I don't know what you mean by "Firefox doesn't enforce [CT]". Mozilla has been instrumental in dis-trusting misissuing CAs.
I don't know what you mean by "Firefox doesn't enforce [CT]". Mozilla has been instrumental in dis-trusting misissuing CAs.