For a more detailed, albeit less well organized, view of functionality yet to be added to the WebExtensions API, there's also the relevant Bugzilla whiteboard [1].
The tl;dr: is that, compared to the XUL API, WebExtensions provide a very limited set of functionality, such that many existing XUL add-ons can't easily be reimplemented in or ported to the new API. On the other hand, it's early days yet, and there appears to be a reasonable amount of interest and developer support devoted to extending the new API so that it offers capabilities comparable to the old. I'd imagine it'll probably be next year this time, more or less, before the dust has largely settled.
The tl;dr: is that, compared to the XUL API, WebExtensions provide a very limited set of functionality, such that many existing XUL add-ons can't easily be reimplemented in or ported to the new API. On the other hand, it's early days yet, and there appears to be a reasonable amount of interest and developer support devoted to extending the new API so that it offers capabilities comparable to the old. I'd imagine it'll probably be next year this time, more or less, before the dust has largely settled.
[1] https://bugzilla.mozilla.org/buglist.cgi?f1=status_whiteboar...