Fireflies is Gustav's event in Spiritfarer. It is indicated on the map as a black splotch with a golden firefly icon. There are three total found. The first is near the Kaltstein Mines in the northwest ice region. The other two are in the far east.
During the event, clouds of fireflies appear on the right side of the screen and move to the left in mostly a straight line. The majority will appear gold in color and reward glim. Occasional red fireflies carrying small square pouches reward Fireglow Seeds.