Garfield Lynns was a movie pyrotechnics expert, but when he lost his job he turned to crime. His passion for fire lead him to arson for hire, and soon he was a full blown pyromaniac. Building a sweet looking suit with wings and a flamethrower he became Firefly, and he pops up in Gotham every once in a while trying to burn part or all of the city to the ground. Wikipedia mentions some stuff about his original theme being lighting effects instead of fire, which was news to me.
Pros: He's got a cool costume that could work in the movie universe. More importantly, Batman dangling from a rope while a guy flies around setting stuff on fire? How cool would that be?
Cons: Sadly, once you get past the costume and the potential for action scenes, the character just isn't all that interesting. The Joker is so much more than a crazy guy in clown makeup, but Firefly is just a crazy guy in a suit.
How I'd do it: He can't carry the movie, but he could be a secondary enemy. Like The Scarecrow in Batman Begins, maybe. Or more like how at the beginning of James Bond movies, Bond foils some evil plot with a lengthy action sequence, and then the plot begins. I could see a Batman movie opening with a great Firefly scene, then a scene of a tied up Firefly in a beat up (and still smoking) suit sitting outside Gotham City PD.
Casting Guesses: I have to be honest, I don't even remember what Garfield Lynns even looks like with the suit off. It was hard to come up with a decent picture for the header. But since in my mind he won't be a major character, he doesn't have to be a big star either. So I'll randomly throw out Callum Blue, who I thought was really good on Dead Like Me.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.