Something changed recently, lets encrypt certificates that are issued by Lets Encrypt Authority X3 doesn't seem to be valid in Mozilla Firefox. It has worked with certificates that I have requested last month that was issued by Lets Encrypt Authority X1.
FIREFOX 45.1 ERROR
www.xxx.xxx1 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
SAFARI : UNTRUSTED CERTIFICATE
According to Lets Encrypt support forum, the intermediate certificate is hardcoded in Dreamhost LetsEncrypt client and that is the source of the problem.
Details available here: