The support for the Application Cache API (AppCache), deprecated since Firefox 44, will be removed in the future in favour of the Service Workers API that enables a richer offline experience. AppCache can still be used as a fallback for legacy browsers, but will be ignored anyway when a Service Worker is registered. Although there’s no specific timeline for removing the AppCache support from Firefox at this moment, it’s a good idea to start learning the new API from now. You can find various useful examples in the Service Worker Cookbook.
Update: The Service Worker API has been shipped with Firefox 44, so it’s now available on the Beta and Release channels as well.