Comment 10 for bug 1737642

Well, it's not impossible. Seems you can just manually set the following
preference in the about:config page:

browser.cache.offline.insecure.enable = true

(It's a pity this still tries to "protect" you from malicious wifi
attackers on the private network blocks like 127.0.0.1 or 192.168.*.*
which are, um, unlikely to be used at all except on fully trusted networks.)

More generally, I can't really get upset about their logic for making
this not available by default. Maybe this is a good opportunity to
provide easily accessible instructions for enabling HTTPS and guiding
the user along the process of importing their own self-signed
certificate into their browser's trust store.

This could probably be automated, since there's already a certgen plugin
that seems to be used just to test the server's HTTPS support. You could
offer a button to generate and use a new SSL certificate, then open the
public component in a file browser and link to general browser
documentation for trusting a new SSL certificate.