This could even prevent man-in-the-middle attacks on scripts that otherwise would never expire anyway like described here: http://thejh.net/written-stuff/want-to-use-my-wifi
I actually even always wondered why the default css applied to elements isn't standardised, so pages not containing reset.css or normalize.css-sheets render differently or how certain Javascript methods differ from browser to browser.
But I guess that is a rather different discussion.
So, different browsers would have different libraries included depending on who made them. Possibly different versions too. It would just be very messy, with little reward.
Using the Extensions API I could even stop the DNS check and inject the Javascript before that which was pretty awesome.
My version would simply just look for cdnjs.cloudflare.com links in the source before rendering but could be applied less strictly to other assets e.g. Google jQuery CDN
(I co-started cdnjs.com a few years ago)
No - release management would be a nightmare on both sides (“Is feature X worth not using the built-in previous version?” “Ooops, new jQuery point release. Time to ship a Firefox update!”) and it offers no advantages over simply using HTTPS to prevent injection attacks and Cache-Control headers to allow saving a properly versioned URL forever.