Extensions is currently the only thing FF is better than the competition at, and they scrap it to copy Chrome APIs... I just can't understand why they would scrap XUL-based extensions before their new APIs reach feature parity. The day pentadactyl stops working is the day I stop using firefox.
The reason to scrape them is simple: the total API surface of XUL-based extensions is pretty much all of the internal APIs of Firefox, which means that any change anywhere in the code of Firefox breaks some extension accidentally. That's a compatibility burden that Mozilla could afford when the only competitor was IE, but it makes the development of Firefox much less nimble than that of Chrome & co. So a new, less invasive and more future-proof API is sorely needed.