Since I don't use firestarter, I can't help you there but since it is just a front end for iptables, why don't you eliminate the middleman and start writing your own firewall? It is actually easy to start with a firewall that drops everything and then add a few accept rules so the stuff you usually do comes through. You can either add the rules in rc.local or set up a separate firewall file and run it from rc.local (or one of the other startup scripts).
A really basic script might be
iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP
iptables -A INPUT -p tcp --syn -j ACCEPT
iptables -A INPUT -p tcp -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT -p tcp -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
That should only allow programs you start on that machine to access the internet and should keep out connections from the outside.
Also, if you use the search button here there are lots of examples of iptables scripts that could be modified to your needs. You should also check out the iptables tutorial at FrozenTux
. It is kind of heavy going but it has everything you need to write your own firewall.