Firefox is preparing huge changes in its internal structure.
https://blog.mozilla.org/addons/2015/08/21/the-future-of-developing-fir…
Becuase of these changes, X-notifier for Firefox cannot keep current features.
Current version is based on XUL/XPCOM. It will be deprecated near future.
It is recommended to use WebExtension API.
It is nearly same as Chrome Extension API.