Symbolism, Imagery, Allegory
Well, duh. Considering that the whole play is happening because Prometheus stole fire, it's not surprising that "fire" is an important symbol.
It's so important that it both starts and ends the play. In very first speech, the god of Power reminds Hephaestus that they're binding Prometheus to the rock because he stole "the gleam of fire that makes all skills attainable" (7-8). At the end, when Zeus begins tearing up the sky and earth as a little preview of the intense suffering he's about to inflict on Prometheus, "fiery twists/ of lightning shine out" (1083-1084).
So what's it all about? Well, fire represents the ability to transform one's environment—and to learn new skills for transforming it. When fire was the exclusive property of the gods, it symbolized the divine power to make the world do what you wanted it to do. When you think about it, that's pretty major. Fire makes us separate from the natural world: because of fire, we can stay up late to write our masterpieces, cook delicious steaks (or veggie burritos), and send rockets to Mars. That sounds pretty godlike to us.
When Zeus's fiery lightning flashes in the heavens at the end of the play, it's both a demonstration of his own power, and a reminder that his power isn't unique anymore. Fire doesn't divide men from gods; it brings them together. Pretty cool.