I’ve a CentOS server, that holds most of my services for my home network, including several Samba shares. Among other equipment I’ve a Xbox(old one) with XBMC installed which is accessing my Samba Shares.
Recently I’ve decided to install the Firestarter firewall, I used to manage an iptables script but got to the conclusion, it’s must easier to have a prebuilt tested and user friendly way to manage it.  So after successfully installing it, I’ve configured the SMB service to be allowed in all lan incoming connections.
My Windows clients have no problem connecting but the Xbox’s XBMC could not found the Workgroup. So after several tries googling the error,  got the solution:
The issue is that firestarter by default does not allow incoming netbios name broadcasts.
The solution is changing the following code:
In /ect/firestarter/inbound/setup add the following line after the Allow Response Traffic entry :
$IPT -A INBOUND -s $NET -p udp -m state --state NEW -j ACCEPT

Be aware that older versions of the XBM, at least versions before 2006 will still have problems finding the workgroup, so upgrade to the latest XBMC.