bridgeing not routing

[Fred Portnoy]

I'm looking at a packet from a packet capture. The packet's IP address was sourced within our LAN, destination a server out on the Internet (it is a tcp ack, part of an ongoing session) The packet's mac addresses were sourced from the inside interface of the firewall and destination to our LAN's core router. Our firewall is operating in bridging mode, however, not routing. It has a management IP address on the inside interface, but that's it. No other IP address assigned.

Under what conditions would an ipfw bridging firewall grab hold of an outgoing packet and send it back, substituting it's own mac address for the source and the inner LAN router for the destination? 

TIA for any insight

Fred Portnoy
Network Analyst
Plymouth State University

"unfettered by edgy modernisms, or classical influences"