How do you catch jynx in pokemon firered?

You can't catch Jynx in Pokémon FireRed.

Jynx is a Generation I Pokémon, meaning it was introduced in the original Pokémon Red and Blue games. Pokémon FireRed is a remake of the original Red version, so it only features Pokémon from Generations I and II.

Jynx was not introduced until Generation II (Gold and Silver). Therefore, it's not possible to find or catch Jynx in Pokémon FireRed.