Comment 3 for bug 1737642

From here: https://developer.mozilla.org/en-US/docs/Web/HTML/Using_the_application_cache :

Deprecated
This feature has been removed from the Web standards. Though some browsers may still support it, it is in the process of being dropped. Avoid using it and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

Using the application caching feature described here is at this point highly discouraged; it’s in the process of being removed from the Web platform. Use Service Workers instead. In fact as of Firefox 44, when AppCache is used to provide offline support for a page a warning message is now displayed in the console advising developers to use Service workers instead (bug 1204581).