It's cross-signed for ubiquity purposes as not everything out there has the COMODO RSA CA as a Root but would likely have the AddTrustExternal Root CA.
The Root version has been included since about Firefox 36 (~2/3 years ago). I suspect your web server is misconfigured in that its not presenting a full chain to the AddTrust as this is very common thing to happen, in my experience.
Have you used
Comodo's SSLAnalyzer to test your site to see if it is trusted?
Have you worked with
Comodo's Support to troubleshoot further? (phone/email/chat)
Did you know that Comodo's CA certificates can be found on their support site?
Here I'd contact Comodo support if you need assistance in installing the CA certificates correctly. They can also help to see if the webserver is presenting the CA certificates correctly & fully.