Badass Normal: When a dragon tries to kidnap a pony in front of her, her immediate response is to hammer-punch it. It doesn't work, of course, but she still tried. She's still tough enough to turn the tables against a lot of powerful baddies.
Berserk Button: Harming innocents beings. When she got past the shock of what happened, she got mad quick seeing Tirak's minions attack the ponies.
Combat Pragmatist: She kind of has to be, given her opposition and Badass Normal status. Tricking a giant crab into losing his balance, blinding the nigh-omnipotent Lavan for a few precious seconds by dropping a drape on his head...
Deadpan Snarker: Briefly, though you'd probably snark too if a pink pegasus picked you out of the blue and treated you like The Chosen One just because you said you could ride horses.
An adventurous, danger seeking pegasus. When ponies start getting kidnapped, she's the one who seeks out help (Megan) and is part of the rescue team. Despite only appearing in "Midnight Castle", Firefly is arguably the most iconic pony of G1 thanks to Wolverine Publicity.
Firefly Publicity: "Rescue at Midnight Castle" was later rereleased as "Firefly's Adventure", despite the fact she's technically not the main character. Firefly is also the de facto G1 pony - she had the most merchandise, was a major player in the British comics, and had a few toy re-releases.
OC Stand In: In the Friendship is Magic fandom, she is often depicted as Rainbow Dash's mom.
I Warned You: Not a spoken example, but boy could she bring out this phrase if she were just a bit less nice. Danger is Firefly's life, and Medley is the one is likely to warn you that a dark, spooky short cut home is not a good idea.
A motherly unicorn who can teleport by wishing, she joins Megan and Firefly on the quest to Midnight Castle.
Chekhov's Skill: Near the beginning of the movie, she teleports down from a cliff to the ground. At the climactic scene, she teleports again to escape Tirac's minions and keep them from getting the Rainbow of Darkness back from her.
Walking Disaster Area: In the first few minutes, she manages to almost drown Baby Moondancer, ruin a banner, and catapult a unicorn and Baby Dragon through the air into buckets of paint. By accident.
Jerkass: Admittedly, we only see her after Sundance has accidentally has ruined a banner she and another pony have worked hard on. Then later catapulted into the air by Sundance which resulted in her being covered her in paint, but she remains very rude and unkind to the poor mare all throughout the special without a single Pet the Dog moment.
Baby Moondancer, Unicorn
Despite an adult Moondancer appearing in the previous special as a background pony, this filly Moondancer gets kidnapped by Catrina.
Distressed Damsel: As early as her first few minutes in the special. She's accidentally knocked into the river by Sundance (I'm beginning to sense a pattern here) and almost drowns. She's later kidnapped by Catrina and Rep.
Catrina's shapeshifting lizard sidekick. Wants to go back to the "good ole days". Voiced by Paul Williams of all people.
Domestic Abuse: When we first meet Rep, he's very poorly treated by his girlfriend Catrina. She treats him more like a servant and batters him about a few times. The show does not treat this like it's okay.
Even Evil Has Standards: He is not evil, but intimidated by Catrina, but he will not go so far as to killing anyone.
Prison Break: When they decide they've had enough of being Catrina's slaves. So they all roll through a big freaking hole in the wall leading directly outside into a forest. The Alcatraz Catrina's abode was not.