Secure Connection Failed

Can you share the rest of the error message which is more important to understand what's happening, incl. any content of "advanced" sections that can be expanded?

This is all I get!

Ah sorry I should have figured out it's the very brief "integrity" error. Usually something like a VPN or some antivirus or malware/security protection is poking its nose into the connection and breaking the trust there…

The frustrating thing is that it isn't a problem using safari on the same computer with the same security protection enabled. It is just Firefox.

Safari uses the system trust store. If you have e.g. a corporate security SW that installs its "man–in–the–middle" certificates on the device to appear legitimate after it deciphers your connections, Safari won't know something's off. Firefox has its own trust roots so it can tell if the secure connection has been tampered with.