The beta.pldt.com server doesn't send the required intermediate certificates needed to build a certificate chain that ends in a trusted root certificate. You can check the server. *https://www.ssllabs.com/ssltest/analyze.html?d=beta.pldt.com<br>This server's certificate chain is incomplete. Grade capped to B. The server needs to send the full certificate chain and include all required intermediate certificates. Firefox will cache intermediate certificates send by a server in the Certificate Manager for future use. If you have visited a website that has send this intermediate certificate in the past then Firefox will use the cached certificate and will not display the error page when you visit a server that doesn't send this intermediate certificate. You can possibly contact the website and ask them to look into this and install the intermediate certificate. Link to certificate: *http://secure.globalsign.com/cacert/gsrsaovsslca2018.crt Do NOT set trust bits on an intermediate certificate when prompted. Trust bits are only required for trusted root certificates and should never be set on an intermediate certificate.

Thank you cor-el for the quick response. I'll take a look at the certificate. Cheers!