Oatmail.io Untrusted Connection Error (FF v18.01)

oatmailerror

  This Connection is Untrusted

    You have asked Firefox to connect
securely to oatmail.io, but we can't confirm that your connection is secure.
    Normally, when you try to connect securely,
sites will present trusted identification to prove that you are
going to the right place. However, this site's identity can't be verified.

  What Should I Do?

    If you usually connect to
this site without problems, this error could mean that someone is
trying to impersonate the site, and you shouldn't continue.

    oatmail.io uses an invalid security certificate.

    The certificate is not trusted because no issuer chain was provided.

    (Error code: sec_error_unknown_issuer)

    If you understand what's going on, you
can tell Firefox to start trusting this site's identification.
Even if you trust the site, this error could mean that someone is
tampering with your connection.
          Don't add an exception unless
you know there's a good reason why this site doesn't use trusted identification.