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