Mewtwo is only available in the games Pokémon Gold, Silver, and Crystal. It's located in the Cerulean Cave, which is inaccessible in FireRed.
If you're looking for a powerful Pokémon in FireRed, you might try these options:
* Trade for one: You can trade for powerful Pokémon with other players.
* Train your existing Pokémon: Focus on training your existing team to high levels and with strong moves.
* Focus on strategy: Learn to use the right moves and strategies to take advantage of your Pokémon's strengths.
Let me know if you have any other questions about FireRed or other Pokémon games!