Well, fireflies in Story of Seasons often show up near the pond or in the grassy fields during the warm summer nights. Just keep an eye out for their little glowing lights.
To find fireflies in Story of Seasons, try exploring the outskirts of the town or the natural areas when it's dark and the weather is nice. Sometimes they can be hidden among the bushes or near the water sources.
You might find fireflies in the forest area during the summer evenings in Story of Seasons.