History of Firefly

Firefly is a Season 4 trait and weapon perk, notably on the exotic Ace of Spades. Precision kills make the target explode, dealing Solar damage to nearby enemies, and precision kills with the weapon also increase reload speed. It is also a perk on some Legendary weapons.

Ace of Spades catalyst

The Ace of Spades catalyst adds the Funeral Pyre perk, which makes Firefly deal more damage while Memento Mori is active. The sources checked give no numeric value for the bonus. Memento Mori gives the fresh magazine a few extra-damage rounds after a reload, and a precision kill with one of those rounds triggers Firefly. To finish the catalyst, get 500 kills with the Ace of Spades; kills made before the catalyst is obtained count toward the total. Sources differ on where the catalyst drops, naming Strikes, Crucible matches and Vanguard playlist activities.