Download and install vCenter Server root certificates to avoid Web Browser certificate warnings
Here in this video we see how to add vCenter root certificate to Ubuntu machine.
This video is helpful in resolving the error "SEC_ERROR_UNKNOWN_ISSUER".
Helpful details for the video:
Rename this file: certs/lin/*.0 to certs/lin/FQDN.crt
Copy the file FQDN.crt to /usr/local/share/ca-certificates/FQDN.crt
Update the CA store: sudo update-ca-certificates