Yes .. I was refering to System prefs > OSX firewall sorry. The image you showed of your LW prefs already shows that port being the one LW is listening through.

But then if using UPnP, that listening port may in fact change from one occasion you open LW to the next. Better check on that.

What brand & model of modem/router are you using? Perhaps it does not have UPnP ability or it's implementation is poor.