First of all, you should really, really reconsider if this is such a good idea to let NBT traffic through your firewall. Especially if the firewall is between your internal network and Internet. 
The problem with NBT is that at once you open it up through the firewall, people will have potential access to all NetBios services, not just a selection of them, such as printing.
The following is a list of the ports used by NBT:

netbios­ns 137/tcp NETBIOS Name Service
netbios­ns 137/udp NETBIOS Name Service
netbios­dgm 138/tcp NETBIOS Datagram Service
netbios­dgm 138/udp NETBIOS Datagram Service
netbios­ssn 139/tcp NETBIOS Session Service
netbios­ssn 139/udp NETBIOS Session Service   _{Your Comment}