Actually, this version of Firefox activates a number of web features that are not activated in the release version, to let webdevs test against features that are either not 100% stable or not fully standardized yet.
It's not revolutionary (we're not releasing Servo yet), but it is pretty fantastic. I'd be shocked if most Firefox Aurora users don't switch over to the developer browser next week. :)
Well, we have had Firefox Nightly (which is the equivalent of Canary) for, well, as long as we have had Firefox. But activating some experimental features for webdevs is new.
But other than that, it's basically Firefox.