The browser storage for HTML Application Cache (AppCache), deprecated since Firefox 44 and already removed from the Nightly and early Beta channels since Firefox 71, is no longer available in all channels as of Firefox 77.
applicationCache property and
OfflineResourceList interface are still exposed on
window to avoid possible site breakage, but the now-useless DOM API will also be removed in the near future. Use the Service Workers API to offer a richer offline experience.
Google is also removing the AppCache storage with Chrome 84.
Update: Just like several other changes, the removal has been postponed from Firefox 77 to 79 due to the COVID-19 pandemic. AppCache remains disabled on the Nightly and early Beta channels.
Update 2: Mozilla plans to make this change with Firefox 80.