It's always a joy to see fireflies -- here in the Carolinas, it's June when we see most of them.
I don't know that much about fireflies -- just that the males flash to attract mates; the periodicity is meaningful; and different species flash at ground level, mid-level, and up in the canopy.
We had a colleague years ago who studied them in the Smokies. He'd head off in June to lie on the forest floor at night and do counts and monitoring. (He had been a city dweller before we knew him, so he seemed an unlikely person to be doing this kind of research!)
Fireflies are definitely seasonal, and hmm, a quick google search brought up this; clearly fireflies are impacted by human disturbance as so many other organisms have been.
But they're still relatively common in the Eastern U.S. and elsewhere in humid areas of the world, apparently.