This is more of a note for anyone else who encounters this today: Firefox is giving sec_error_ocsp_unknown_cert error when trying to access gtdnex.com or app.gtdnext.com today since the SSL certificate was updated yesterday. It looks like this is a reasonably common issue with renewed StartCom certificates and will, hopefully, resolve itself in the next day or two when StartCom updates their OCSP responder list.
It’s possible to disable OCSP checking in Firefox, which would bypass this issue, however I would recommend against doing this since it’s a global setting.