You/They can check the status using a neutral testing site like: https://www.ssllabs.com/ssltest/ ''If that shows all is well, and the problem is only in Firefox:'' Your connection might be proxied in some manner, or the server/CDN might be serving different files to Firefox than other browsers. ''If that shows the cert was revoked:'' If they host their site on GoDaddy or have a cert issued by GoDaddy or one of the other companies they own, this is not an uncommon issue that their cert is mysteriously revoked and reissued. The reason other browsers don't report this immediately is that instead of using an OCSP lookup, they rely on a static list (CRL) downloaded from time to time. They'll want to contact their cert issuer for updated files.