Firefly is an adventurous, danger-seeking pegasus pony and the Main Protagonist of the My Little Pony pilot episode Rescue at Midnight Castle. When the Scorpan and his dragons kidnapped some of her friends, she flew to find help and found Megan. She brings the human girl back to Dream Valley while encouraging her that she's special. She joins the rescue team, which includes Megan, Twilight, Bow Tie, and Applejack.
Despite her sole appearance in Rescue from Midnight Castle, she's the most iconic character due to having the most merchandise. She was a major character in the UK comics.
She was to appear in My Little Pony: The Movie. There was a never-animated scene where she flies over the Rainbow Bridge to find help. She encounters Shipwreck and Optimus Prime before finding Megan. The aforementioned G.I. Joe scene involves a drunk Shipwreck seeing her while drinking, before proceeding to smash his whiskey bottle and start praying to himself. Hasbro's response to the scene: "Very funny... no".
She was the inspiration for Rainbow Dash in My Little Pony: Friendship is Magic. She is creator Lauren Faust's childhood toy and in fact, originally wanted her as one of the main ponies, but Hasbro lost the trademark to her name. So Lauren resigned the character, giving Rainbow Dash her G3/G3.5 counterpart's name and colors, but with a cutie mark completely different from either Firefly and G3/G3.5 Rainbow Dash.