Once upon a time, Kevin Kofler kevin.kofler@chello.at said:
Most likely it's just a self-signed SSL certificate. Very common, and Firefox stupidly throws a fit over it (which is dumb because it encourages sites to just use unencrypted HTTP instead, which is even less secure, yet gets through with no warning). Just OK the certificate.
HTTPS with an unknown self-signed cert is barely any more secure than unencrypted HTTP, since a man-in-the-middle attack could just be replacing the cert and decrypting all communications.
However, the reason to "throw a fit" is that end-users have been trained that "HTTPS == secure". They know that HTTP is not secure, but they don't know the details of how SSL/TLS work to know that "HTTPS with unknown cert != secure".