Unfortunately, as far as I can tell, napshare doesn't play nicely with firewalls at all; it doesn't allow you to specify the local port range it uses for incoming connections. Therefore, when napshare binds a local socket to listen for the incoming file transfer, the OS picks a local port out of the ephemeral port range.

Which means, essentially, if you want napshare to work behind a firewall, you need to open the entire ephemeral port range. You can tell what the range is by viewing the contents of this /proc file:

/proc/sys/net/ipv4/ip_local_port_range

IMO, napshare desperately needs to provide a way for the user to select the local port range used for incoming connections.