Currently, fwbuilder using iptables does not support generating rules with netmasks like 255.0.0.248.
10.0.0.248/255.0.0.248 would translate into all addresses starting with 10. and ending with IPs from 248 to 255. This would be a great feature to define i.e. router-IPs in all available networks behind a firewall. If a networtk is added, the previously generated rule is still valid, as the new router-IPs are within 10.0.0.248/255.0.0.248.
Just a suggestion.