I had this problem lately, that firestarter crashes continuously and what i get was something like this

root@0x80:~# firestarter
Firewall started
Adding Firestarter startup hook to /etc/dhclient-exit-hooks
Firewall started
 
***MEMORY-ERROR***: firestarter[3290]: GSlice: assertion failed: sinfo->n_allocated->0
Aborted

and after doing some searching, i found that its a GUI-lack in its code, and i think specifically in GUI lists box
because each time you open a listbox, you’ll get a MEMORY-ERROR its something related to allocating memory size
anyways as far as i know there is no patch for this bug but you can solve it by adding using G_SLICE environment variable.
G_SLICE allows you to reconfigure GSlice() memory allocator.
Set G_SLICE variable to always-malloc , this will make all slices allocated to be actually allocated via direct calls to g_malloc() and g_free().

qnix@0x80:~# G_SLICE=always-malloc gksu /usr/sbin/firestarter

This will start firestarter with our G_SLICE via gksu (GTK+ frontend for su and sudo), by the way
don’t use sudo because it might not pass the G_SLICE to the su environment. or if you really need
to use sudo just edit /etc/sudoers and checkout the Defaults option it should look something similary
to this

Defaults        !lecture,tty_tickets,!fqdn

change it to

Defaults        !lecture,tty_tickets,!fqdn,env_keep+="DISPLAY G_SLICE HOME"