Firefly is an accessory in Final Fantasy XII that reduces EXP gain to zero for the wearer. It can be used to avoid leveling up, such as to avoid a boss's level-based attacks, or for a challenge playthrough.
In the original PlayStation 2 versions, Firefly is bought from Phon Coast for 5,000 gil. It is found as a treasure in Stilshrine of Miriam's Walk of Reason (northwest; 55% chance to appear, 45% chance for gil, 50% chance the item is a Firefly without the Diamond Armlet ), and Cerobi Steppe's North Liavell Hills (50% chance to appear, 65% chance for gil, 10% chance the item treasure is a Firefly with the Diamond Armlet equipped ).
Fifefly is a rare drop from the Slaven Wilder. It is made in the bazaar in the "Blush of Light" set by selling one Magick Lamp (from Mallicant), two Tomato Stalks (from Topstalk), and one Snowfly (from Nightmare). It is likely not worth trying to make Firefly purposefully in the bazaar for the accessory's niche use and availability to simply purchase from Phon Coast.
It is also a possible reward for the Ktjn sidequest in Rabanastre. She gives the Firefly as a prize when she gets the -4 score from the conversations with the player, and settles inside Migelo's shop.
In the updated Zodiac versions, Firefly is bought from Dalmasca Westersand (after obtaining the Treaty-Blade) for 1,600 gil. It is a treasure in Dalmasca Estersand's Outpost (always appears, so the player can get one right at the start of the game if desired), Zertinan Caverns' Invitation to Heresy (always appears), and Subterra's Penumbra - North (70% chance to appear, 80% chance for gil, 50% chance the item treasure is Firefly without the Diamond Armlet), as well as in Trial Mode stages 89 and 97 with the Diamond Armlet equipped.
It is still a rare drop from Slaven Wilder and is made in the bazaar in the "Blush of Light" set with the same requirements as before. Firefly is still a possible reward for the Ktjn sidequest, now the reward for the -3 score from the conversations when Ktjn appears outside Migelo's shop at the end of her quest.
Firefly is an accessory with a niche use. It stops the wearer from gaining EXP, preventing any level ups. It can be used for a challenge playthrough, though the Zodiac versions already have a "New Game Minus" mode where the player cannot level up. It can also be used to better manipulate the party's levels for an upcoming boss that uses level-based abilities, like Gilgamesh.
Firefly is available much earlier in the Zodiac versions, making it more usable for a challenge playthrough.