Luiga is a Pokemon introduced in Generation VI (Pokemon X and Y) and is not available in Pokemon FireRed (Generation III).
If you're looking to catch a legendary Pokemon in FireRed, you can find:
* Articuno, Zapdos, Moltres: These legendary birds are found in the Seafoam Islands.
* Mewtwo: Found on Cinnabar Island after defeating the Elite Four.
* Mew: This elusive Pokemon can be found in the game's code, but it's not possible to encounter it naturally.
If you're interested in catching Luiga, you'll need to play a game from Generation VI or later.