I'm by no means an expert, but I'll try and help. I suppose we're talking the IPTABLES firewall here. In a console with root rights try:
"iptables -A FORWARD -p ICMP -i ETH1 -o ETH2 -j ACCEPT" for enabling ETH1->ETH2 and
"iptables -A FORWARD -p ICMP -i ETH2 -o ETH1 -j ACCEPT" for ETH2->ETH1
If the packets are also meant for the linux machine itself, do the same commands, only with INPUT instead of FORWARD. You should also turn port forwarding on in oreder for the pacets to be traversed trough.
I also suggest that you study the man pages for iptables, so you know what you are doing.
Try typing "man iptables" into a console, it's written pretty nice.
Hope I helped, since IPTABLES is among the very few thing I've been able to undrestand so-far.