The HTML5 Application Cache API (AppCache), that enables offline support in Web applications, is now considered deprecated and will be removed from a future Firefox release. Web developers are encouraged to learn Service Workers as an alternative solution, so that you can offer a richer, more seamless offline user experience sometime soon. This new API is already available on Firefox Developer Edition for testing, though not all the features are implemented yet. Note that AppCache is ignored when a Service Worker is registered.
Update: The Service Worker API has been shipped with Firefox 44, so it’s now available on the Beta and Release channels as well.