Hi

Have you considered updating to the most recent version of Firefox?

You appear to using a version which is very close to being three years old.

Note that in such an old version you would likely have to do this directly via the about:config page as this hasn't been added to the user the settings page, but I do not know if there are unresolved issues with HTTPS-Only so use at your own risk if you do not want to update.

• about:config => dom.security.https_only_mode