I'm getting the same problem. If I start from the terminal I see this warning, maybe it has nothing to do with the problem but maybe it will help.
gksu firestarter
***MEMORY-WARNING***: gksu[8552]: GSlice: g_thread_init() must be called before all other GLib functions; memory corruption due to late invocation of g_thread_init() has been detected; this program is likely to crash, leak or unexpectedly abort soon...
Firewall started
I'm getting the same problem. If I start from the terminal I see this warning, maybe it has nothing to do with the problem but maybe it will help.
gksu firestarter
***MEMORY- WARNING* **: gksu[8552]: GSlice: g_thread_init() must be called before all other GLib functions; memory corruption due to late invocation of g_thread_init() has been detected; this program is likely to crash, leak or unexpectedly abort soon...
Firewall started
Firestarter then crashes after a few minutes