Change 7462: "Implement extension:// protocol". Extensions begin!
A bit of history relevant to extensions: Aaron released Greasemonkey right around the time he was hired at Google, so one of those events didn't trigger the other*. Greasemonkey seems almost irrelevant in the presence of Firefox's capable extensions system but Aaron, whose background was view-source web development, was primarily concerned about weight: the more lightweight a system is, the easier it is for people to tinker and rapidly iterate and that has a direct result in the quantity and variety of applications. The popularity of Greasemonkey is a testament to that idea.
So if there is any goal of the extensions system (beyond all the "musts", such as "must be secure", "must be useful", etc.) I like to think it is "be lightweight". I'm eager to see how it develops.
* Beyond wanting to get the current project out of the door because a new one was starting.