Thanks for posting! Since the Ethernet interfaces on the 6310 are routed interfaces, you cannot have the same subnet (broadcast IP address) exist on both interfaces. You do have a few different options though.
1. One alternative would be to split, for example, a /29 subnet into two /30 subnets. In this mode, you would need the upstream router be aware of this change and include a static route to the Adtran unit for the smaller subnet where the firewall resides.
2. If it is not possible to add a static route on the upstream router, you may be able to use Adtran's "ip proxy-arp" feature so that it responds to ARP requests for the firewall behind it. As with option 1 above, you would need that "smaller" subnet to reside on the second ethernet port.
3. You can create a private subnet, such as 192.168.1.0/24, between Adtran unit and the firewall. You would then need to setup NAT on the Adtran unit. The disadvantage is that the firewall cannot be accessed directly, so you will likely need to configure port forwarding on the Adtran unit.
In my opinion, the first option would be the best solution. If you are unable to make this work as you would like, you may need to call in a normal support ticket using 888-423-8726. We would be glad to assist you.
I went ahead and flagged this post as "Assumed Answered". If the response on this thread assisted you, please mark it as Correct or Helpful as the case may be with the applicable buttons. This will make it visible and help other members of the community find solutions more easily. If you still need assistance, I would be more than happy to continue working with you on this - just let me know in a reply.