Fireflies are nasty bugs that need 2 matches to open their wings. One match more and they will close again. When all flies have opened their wings they will all fly away and score at the same time. It's not possible to score a single firefly.
They have also arrived in the Let it Snow event on 2015 Christmas Farm Heroes Saga game on mobile, beginning on level 13.
Are you more of a visual learner? this video will tell you how to collect fireflies in Farm Heroes Saga.