I get the same result. I have tried adding the domain to the following preference in about:config to no effect. network.trr.excluded-domains The following are FYI only as they don't seem to be relevant.

https://wiki.mozilla.org/Trusted_Recursive_Resolver#DNS-over-HTTPS_Prefs_in_Firefox https://support.mozilla.org/en-US/kb/firefox-dns-over-https https://support.mozilla.org/en-US/kb/dns-over-https-doh-faqs

See also https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

The website works for me via an open HTTP connection, so try to create an "off" exception for HTTPS-Only Mode via "Settings -> Privacy & Security -> HTTPS-Only Mode -> Manage Exceptions".

• https://support.mozilla.org/en-US/kb/https-only-prefs

• http://schoolgateway.westminster.org.uk/?page_id=7

Thank you, Terry.

Cor-el: thank you. I also found that the problem went away when I turned off HTTPS-only mode altogether, so this feature is clearly the problem.

Note that exceptions do not work in Private Browsing mode and in PB mode the only option is to disable HTTPS-Only Mode entirely as far a I managed to get it working in PB mode.

Cor-el: thank you again, much appreciated. However, I do not think that work-arounds are good enough here. Firefox is saying that the website is unavailable, which it isn't. It should be saying that the website does not offer a safe connection via HTTPS and should allow the user to proceed if he or she wishes. It is clearly a bug, and I have now reported it via Bugzilla.

For reference:

• 1821012 - "requested service is temporarily unavailable" message in HTTPS-only mode

This is a problem with how the server responds to a secure connection. If there is a response then Firefox assumes the HTTPS is supported.