=== I already have a XUL extension. How is this not terrible news for me? ===
Talk about how There is certainly some short term pain that we want 're inflicting, but we hope that the long term gains are worth it. In the short term (~18 months) you will need to rewrite your add-on to use the new APIs. However, after that, you're done! The intention is for the WebExtension API to be stable enough that addon authors can rely on it not to break extensions lessbetween Firefox releases.
=== Why deprecate XUL now? Firefox is still using it internally. ===