How do you get jiroxy on Pokemon FireRed and leafgreen?

You can't get Jiroxy in Pokémon FireRed and LeafGreen. Jiroxy is a Pokémon that was introduced in Pokémon Sword and Shield, so it's not available in older games like FireRed and LeafGreen.

It's important to note that the Pokémon available in each game are determined by the game's development and release date.