As part of the ongoing insecure HTTP deprecation, Firefox 60 Nightly and early Beta/DevEdition will no longer allow non-HTTPS sites to use the application cache (AppCache). When disabled, cache manifest files will be ignored and window.applicationCache will be undefined . Assuming no major issues arise, Firefox 62 will make the change on all the channels.

Note that the AppCache support itself has also been deprecated since Firefox 44 and will be completely removed in the future in favour of the richer Service Worker API.

Update: As planned, all the channels impose the restriction as of Firefox 62.