Jeffrey,
First go to Google and search for ICMP codes. Find a site that explains the ICMP codes and book mark it. It becomes a valuable reference when reading logs.
The type 3, code 1 message is "destination unreachable, host unreachable". It can come from many things, including a port scan. You should next try and figure out if the source or destination is in your network. WHy are you seeing this at the firewall? Is the source / destination always the same IP or a range of IPs?
Liberty for All,
Brian
Brian Ford | brford@cisco.com | brford@yahoo.com | 51 75 61 6c 69 74 79 20 6d 65 61 6e 73 20 64 6f 69 6e 67 20 69 74 20 72 69 67 68 74 20 77 68 65 6e 20 6e 6f 20 6f 6e 65 20 69 73 20 6c 6f 6f 6b 69 6e 67 2e | Email me when you figure this out.