However, in FireRed, you can learn a similar move called "Light Screen", which creates a barrier to reduce the damage from special attacks. You can teach this move to certain Pokémon by using TM31.
Here's how you can get TM31:
1. Defeat the Elite Four and become the Champion.
2. Head to the Pokémon League in Indigo Plateau.
3. Go to the second floor and speak to the man sitting in a chair. He will give you TM31.
While Light Screen isn't Flash, it can be a useful substitute in FireRed for battling Pokémon that rely heavily on special attacks.