Please describe what you mean by "no longer allows"? If you are using Comcast's webmail, do you have the older "Zimbra" webmail ("web.mail.comcast.net/zimbra/..."), or the newer Open-Xchange "Appsuite" webmail ("connect.xfinity.com/appsuite/...")?
What happens when you try to forward? Any error messages? What browser are you using? The more details you can provide, the better.