An Acronym for Explicit Congestion Notification.
From the Linux configuration help
Explicit Congestion Notification (ECN) allows routers to notify clients about network congestion, resulting in fewer dropped packets and increased network performance. This option adds ECN support to the Linux kernel, as well as a sysctl (/proc/sys/net/ipv4/tcp_ecn) which allows ECN support to be disabled at runtime.
Cisco IOS was fixed in version 12.2(8)T according to this page of Sally Floyd's (one of the authors of ECN) - http://www.icir.org/floyd/ecn.html
This version of IOS was released around 2002 or 2003 so if a firewall is still running that version it probably is very insecure too and should be upgraded.
# echo 0 > /proc/sys/net/ipv4/tcp_ecn