"A major challenge we face is that many Firefox add-ons cannot possibly be built using either WebExtensions or the SDK as they currently exist. Over the coming year, we will seek feedback from the development community, and will continue to develop and extend the WebExtension API to support as much of the functionality needed by the most popular Firefox extensions as possible."

So to all extension developers leaning on the current permissive features or XUL/XPCOM, please contact Mozilla and help them with finalizing their WebExtension API in the upcoming year.

Also, in the future if all XUL in Firefox is replaced by browser.html, there might be full customization options again based on html/css/js (thanks to nwah1 for pointing that out).