The third link in that blog post, and the first "this is what we're
doing" sentence, links to a wikimo page. At the top of that page are a
series of bug links. If you click the "All" link the 2nd earliest bug
(the first is unrelated) is the kick-off bug, filed months before that
blog post.
https://bugzilla.mozilla.org/show_bug.cgi?id=1161828
There were also discussions about the Chrome Extension API in the
mozilla.dev.extensions newsgroup prior to that effort.
-Dan Veditz