Without it being an established W3C standard I think it would be disingenuous to integrate it into the browser. Despite its obvious benefits, it would feel like they're using their browser as a platform to push other products. Until it saw widespread adoption I think implementing it as a browser extension/add-on would be more appropriate.
w3c standards are for what gets rendered by Gecko. The browser itself has many things that do not involve w3c at all (search box, bookmarks, sync, etc.)
