Pokemon FireRed/LeafGreen legendaires

Articuno
Articuno is found in the Seafoam Islands

Zapdos
Zapdos is found at the Power Plant

Moltres
Moltres is found on Rainbow Island 1

Mewtwo
Mewtwo is found in the Unknown Dungeon

Raikou
Roams Kanto after you beat the Elite Four if you picked Squirtle

Entie
Roams Kanto after you beat the Elite Four if you picked Bulbasaur

Suicune
Roams Kanto after you beat the Elite Four if you picked Charmander

Ho-oh
Is found on Rainbow Island 8

Lugia
Is found on Rainbow Island 8

Deoxys
Appears on Rainbow Island 9 after you complete a puzzle