[Samba] Error when subscribing to list

[jamrock]

When I try to subscribe to the Samba mailing list at
https://lists.samba.org/mailman/  I get the following message in Firefox:

"Secure Connection Failed
lists.samba.org uses an invalid security certificate.

The certificate is not trusted because the issuer certificate is unknown.

(Error code: sec_error_unknown_issuer)

* This could be a problem with the server's configuration, or it could be
someone trying to impersonate the server.

* If you have connected to this server successfully in the past, the error
may be temporary, and you can try again later."

Is this normal?