Note, I have only tested this with server certificates signed by a certificate authority trusted by Netscape. This attack may be less powerful if the malicious server certificate is merely self-signed. Furthermore, the security community has many examples showing that DNS is not secure at all. For instance, a teenager recently defaced the RSA.COM site by an attack against a DNS server. It should be trivial to attack targeted individuals and not difficult to attack general users at large.
Users just clicks.
Hide the warning window with a pop-up window. Respond to DNS requests since UDP responses are easily forged. Otherwise, you will have to manually inspect server certificates in Netscape. PSM appears to manage certificates more securely. I was able to install PSM successfully in Linux and Windows98, but sometimes the installation script would fail from a yet undetermined cause.