Mozilla will be moving the interface from XUL and XBL to CSS and then optimizing. It's a logical move as XUL is outdated, archaic, and difficult to learn to use, in addition to not being a web standard. Whether or not the new CSS ui will have similar functionality to XUL is undetermined. There's been a decent bit of discussion about it on MozillaZine but truth be told, things do look grim for Firefox.
Some additional reading:
Firefox Addon Changes: https://billmccloskey.wordpress.com/2015/08/21/firefox-add-on-changes/
Hooray for webextensions: http://robert.ocallahan.org/2015/08/hooray-for-webextensions.html