There are no Legendary Fire Birds in the Pokemon universe, and the only Legendary Birds available in FireRed are:
* Articuno (Ice/Flying)
* Zapdos (Electric/Flying)
* Moltres (Fire/Flying)
To catch these birds, you must first defeat the Elite Four and become the Pokemon League Champion. After that, you can access the Seafoam Islands and follow the story path to encounter them.
Remember, you can only capture one of them, so choose wisely!