Well, honestly, I was hoping someone else would pick up this thread. I don't do Firestarter. Not that I know of anything bad about it, I just prefer to handle rules myself.
The only decent Firestarter info I can find is at the Firestarter Sourceforge page
In particular, you might want to click on 'mailing list' at the top and post a question there. You might also try a post here in forum54 (Linux Server).
Having said that, I did poke around at the Firestarter mailing list a bit myself. It looks to me like Firestarter will use either ipchains or iptables. I would hope you are using iptables. If so, you would need to add rules something like:
$IPTABLES -A INPUT -s xxx.xxx.xxx.xxx/24 -i $EXTIF -p 47 -j ACCEPT
$IPTABLES -A OUTPUT -o $EXTIF -p 47 -j ACCEPT
You will need to change the $IPTABLES to whatever variable your script uses to point to iptables, change the $EXTIF to whatever interface is connected to the internet, ie eth1, and change the xxx.xxx.xxx.xxx/24 to the network/subnet that you are connecting from. You might want to remove the whole -s xxx.xxx.xxx.xxx/24 until you get up and running.
Now, the problem is, I am not sure where to put it. I didn't have time to dig through the stuff at sourceforge to find where firestarter puts the script. Also, that is a iptables rule, it will need to be modified to work with ipchains if that is what you are running.
Make up your own disclamer and insert here. I don't know anything, proceede at your own risk, gremlins might destroy your computer if you do this, etc.
Hope this gives you some idea where to go from here.