AFAIK, Firefox has not introduced any change in order to score 100% on Acid3. If you read this article (http://browserfame.com/212/firefox-acid3-test-100), you will come to know that the developer of Acid3 test has introduced few changes in order to "update" the test. On the updated Acid3 scale, Firefox scores 100%. This simply indicates that Mozilla was correct!
I agree with you: "If you take a Firefox 16 months old and run Acid3 on it now, it would score 100% - I think that's what he meant.".
I am unable to understand your comment: "read before you respond stupidly.". Can you please explain my stupidity?
(FYI: I am also associated with Mozilla for long, as a volunteer.)