Earlier quoted context omitted.
Well, XUL-based extensions are not scraped yet. They will, at some point in 2017, but we're not there yet. 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 w…
Surely the burden is on the extension developers and not Mozilla?
1/ either try and locate all the add-ons that will be broken, get in touch with their developers, be ignored by most of them, start several weeks of negotiation with those who do answer, then eventually, several months after my code is ready, land the change, and notice that Chrome has landed that same change a few months ago;
2/ ignore the add-on developers, improve Firefox immediately, but certainly break some add-ons, hence breaking the user experience of millions of users for no understandable reason.
As you can imagine, neither solution is good and everybody suffers from either.
WebExtensions condense all the instances of 1/ into a single point of time, hoping that we never again need to go through this painful dance.