Fireflies and firefly jars can be found throughout the Fortnite Chapter 3 map. Fireflies in their natural form are most commonly found around campfires and trees, particularly those near rivers. You are also more likely to find them at night. This is partly due to the soft glow they emit, which makes them easier to spot in the dark. Once you’ve found some fireflies, interact with them to add a jar of fireflies to your inventory. Ready-to-go Firefly jars can also be found almost anywhere you can find loot. They are rare, so you are not going to find exactly a surplus, but it is still quite common to find them.
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If you are collecting fireflies for a mission, you need to collect the fireflies. Then you can discard the firefly jar if you don’t want it. However, they can be quite useful weapons, especially if you are up against players who build a lot. This is because it is essentially an incendiary grenade and is excellent for burning enemy structures. However, those structures have to be made of wood, of course. Firefly jars are ineffective against stone and metal built structures.