As far as I'm aware, 57 won't include the XUL API at all, whether behind a config flag or otherwise. I'm not sure how updates from 56 will be handled - one hopes users on the Release channel, who have add-ons the update will break, won't just find themselves hosed by surprise. I haven't heard much of anything about how that actual process will be managed, though.

(Self-Destructing Cookies looks like it could be reimplemented on the WebExtensions API, but the dev doesn't seem to have any interest in doing so. That's a shame, but it doesn't mean no one else will do it.)

I don't blame Mozilla for pulling the trigger this way, though. Opening the roadmap for general community discussion and veto would result in endless bikeshedding, and users unwilling to abandon XUL add-ons in November have the option of sticking with 52 ESR or just not updating past 56 for a while. Conversely, waiting for feature parity with XUL would mean probably another three or so years before e10s hit stable, and Firefox is already three or more years behind in that regard - that much more delay might well not be survivable.

It's not a great situation to be in, but I can't see how Mozilla could've found a better option than the one they chose, and I say that as one who will lose some cherished add-ons in the transition - but if it's that or switch to Chrome because Firefox is too slow to be usable, I'll take the former, and I was strongly contemplating the latter before e10s became a thing. I doubt I was the only one.