I think culture/where you grew up can play a part in whether you say "lightning bugs" or "fireflies". While in college at WKU, I once had a bit of a culture war with my roommates. All in good fun, promise. One roommate was from SoCal, the second from TN, and myself from Georgia. I remember my SoCal roommate once said, "Oooo! Look! Fireflies!" My TN roommate and I were both like, "What? Where?" We then proceeded to teach the SoCal roommate that, in the South, we call them, "lightnin' bugs". (Spoken with Southern accent, of course.) We also taught her some other important things, like the appropriate way to say, Louisville. ("Lou-ah-ville", not "Louie-ville" or "Louis-ville".)