I have a valid letsencrypt certificate for my NS 7.3 server installed and default.
I show a secure connection in Chrome, IE, Edge, but in Firefox I get the following error;
office.mydomain.com uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. The server might not be sending the appropriate intermediate certificates. An additional root certificate may need to be imported. Error code: SEC_ERROR_UNKNOWN_ISSUER
Looking at the certificate in Firefox shows,
Could not verify this certificate because the issuer is unknown Issued To: Orginazation (O) Orginazation Unit (OU) Issued By: Orginazation Unit (OU)
I used the Server Certificate section of the NS web interface to request the LE cert.