The "XUL Interface" is largely a misnomer. Firefox has had extension interfaces that are compatible with multiprocess firefox for years. Yes, you can break compatibility, but if you do it right it will keep working. Also the browser will tell the extension that it's not allowed to do that so the extension developer will know to fix their extension.
Don't get me wrong. I love the webextension idea. But it doesn't replace the ability to have extensions with the same power as the browser itself. I would love for Mozilla to promise compatibility for webextensions and say "you are on your own" for traditional extensions. They can mark them as bad in the store, even refuse to fully review them so that I know that they are why my browser is broken. But sometimes that power is incredibly useful, and I would hate to see that taken away.
Like many others the only reason I use firefox over chrome is because of a couple of extensions and if they were gone I would switch because internal sites at my work tend to break less and internal extensions generally only support chrome.