Captive Portal & Firefox

Hello Everyone, glad to be here and hope someone can give a bit of help on a problem I can't seem to get. Been trying to use Captive Portal but keep running into trouble with firefox. I've pushed out the Untangle root cert via gpo and it works fine, for other browsers, however I keep getting an "issuer chain not provided" for firefox.

I've looked up that it could be an "intermediate certificate" that is missing, but i'm a bit unclear on how to find that.

Thanks for any help.