What do you if killed the three legendary birds in Pokemon fire red?

In Pokémon FireRed, if you kill the three legendary birds (Articuno, Zapdos, and Moltres), you will not be able to progress further in the game.

You will be unable to enter the Hall of Fame, and you will not be able to catch any more legendary Pokémon.

Additionally, you will not be able to access any of the areas that are unlocked by defeating the three legendary birds.

If you have killed the three legendary birds, you will need to restart your game in order to continue playing.