Mewtwo is considered to be the final "boss" of Pokémon FireRed, and thus, there is very little structured content progression after capturing and/or defeating Mewtwo.
However, this doesn't mean the game is over. You can go and begin EV training your Pokémon if you want to battle other players competitively. You can also look to complete your Pokédex, but that may prove impossible without a Gameshark or other cheating device. If you do happen to have a cheating device, you can go explore the Sevil Islands for a bit more storyline as well as a few other battles with Legendary Pokémon.