rfc 2979…

http://www.ietf.org/rfc/rfc2979.txt

your firewall hates icmp…

   ... it's proper (albeit unfriendly) to block ICMP Echo
   and Echo Reply messages, since these form a different use of the
   network, or to block ICMP Redirect messages entirely, or to block
   ICMP DU/FN messages which were not sent in response to legitimate
   outbound traffic.