By Firefly's Light is a side quest in The Legend of Zelda: Breath of the Wild. It is a quest given to Link at Kakariko Village. The objective is to capture five Sunset Fireflies and let them all loose inside of Lasli's house in order to complete the quest. There is an abundance of Sunset Fireflies in Kakariko Village, but Link must be careful not to spook them off.
This side quest is part of a trio of quests that need to be completed in order to trigger the Shrine quest "The Stolen Heirloom", in order to find and activate the Lakna Rokee Shrine, the two other being the main quest "Find the Fairy Fountain" and the side quest "Flown the Coop".