Starting with Firefox 62, the HTML5 Application Cache (AppCache) cannot be used any more on non-HTTPS sites, where cache manifest files will be ignored and
window.applicationCache will be
undefined. The security restriction has already been introduced to Firefox Nightly and early Beta/DevEdition since the version 60.
Remember that AppCache has been deprecated since Firefox 44 and will be completely removed in the future in favour of the richer Service Worker API, which only works with HTTPS sites from the beginning.