Flash Fire is a Pokémon ability introduced in the third generation. It causes fire moves to power up the user.
Pokémon with Flash Fire are immune to Fire attacks. If they get hit by a Fire-type attack, they get a 50% increase in Attack or Special Attack that only applies to Fire attacks. In Generation V forward, the ability now works even if the user is frozen. However, if the user is never actually hit by the fire attack, they do not get the boost.