Lots of good points about Garlean stuff in here. As for the Eorzean city states, according to this early quest Limsa uses fish (or fishy-smelling) oil as fuel for streetlamps, and I at least assumed the others would use something similar (as with real-world pre-electric lighting) for their streetlamps and waymark-lanterns. As for what actually lights them, I don't think it's stated anywhere. It could be a fire or lightning crystal I suppose, or just a match. *shrug*