Fireflies are beetles, and there are two varieties: luminescent and non-luminescent. The book Colorado Nature Almanac states that:

"Each species of lampyrid beetle has its own unique light signal which varies in color and timing...British scientist Thomas Mouffet was less romantic. He wrote that fireflies '...being drank in wine make use of lust not only irksome but loathsome...' (129) (quoted in Evans, Life on a Little-Known Planet).

That seems a little harsh. Mouffet probably wouldn't sit on the front porch in the summertime and watch the fireflies flash. Here's a more romantic notion:

"In the Philippines they say fireflies received lanterns as a reward for finding jewels lost by the insect king..." (129).

The main ingredients of the cold light of fireflies is luciferin, luciferase, and adenosine triphosphate. The word Lucifer means light- bearer in Latin, but Lucifer is usually thought of as Satan in modern times. Rather than being pictured as a horned monster with a tail, hooves, and sometimes a pitchfork, the Old Testament pictures Lucifer as a bright, shining prince whose presence was set among sparkling jewels. The horns and tail often attributed to the more recent depictions of the Devil are derived from pagan cults whose gods and goddesses were often part or fully animal. Those gods were usually explicitly licentious and viewed as fertility symbols. Don't blame me for this.

At any rate, despite Mouffet's dismal view, people do enjoy firefly flashing on a summer's evening as the dark is lit as if by twinkling stars, and the sky seems not above us but all around us, we among the stars.