Solution for: #67: Unknown password when importing a client certificate in Firefox

So Firefox appears to require certificates that are in PKCS12 format (.p12) when importing. It cannot determine whether a client certificate file is encrypted, or if it is simply not a PKCS12 format file. My certificate, as downloaded from StartSSL, was in .cer format. Firefox would keep prompting for a password that doesn't exist.

The solution was to export the certificate in PKCS12 format, upon which point it was accepted by Firefox without triggering the password dialog. I was able to do format conversion via Keychain Access.app in OS X.