Is mewtwo in the pokemon mansion in FIRERED?

Yes, Mewtwo can be found in Cerulean Cave in Pokémon FireRed, not in the Pokémon Mansion. The Pokémon Mansion, located on Cinnabar Island, contains various other Pokémon and items but does not house Mewtwo. To encounter Mewtwo, players must first defeat the Elite Four and then navigate through Cerulean Cave, which requires Surf to access.