Webserver using older ciphers which are not supported by firefox like sslv3.
The certificate is not installed properly.
The port is not open for https connection
Check and confirm if the webserver on which you have installed the certificate has the most latest protocols (make sure sslv2 & sslv3 are disabled)
Check the certificate configuration and confirm if the complete chains of certificate (domain+intermediate+root) certificates are installed. Moreover, confirm If the private key is matching with the public key
Make sure the port which is configured to accept the https connection of the respective URL is open.