I'm using pfSense and FreeNAS locally. There's no DNS or LAN-wide CA service, devices use NetBIOS/Samba for shares and standard private IPv4 addresses otherwise. Both pfSense and FreeNAS provide a web-based GUI and both allow the use of HTTPS (pfSense automatically generates a GUI cert, FreeNAS has CA+cert pages for the user to do the same).
In Firefox the pfSense https GUI is correctly detected as insecure (no public/verifiable chain of trust) with an option "Add exception", but the FreeNAS https GUI is described as "Secure Connection Failed" with no details and no option to add an exception.
As the two systems and their network environments are functionally identical AFAIK for certificate/GUI purposes, clearly the issue is likely to be related to the FreeNAS certificate or CA settings I set up being inadequate or incorrectly configured.
But how can I find out what the key difference is, or fix it? What is the problem likely to be?