Where can you catch a haunter in Pokemon FireRed level 25?

You can't catch a Haunter in Pokemon FireRed at level 25.

Here's why:

* Haunter is a level 25 Pokemon. This means it evolves from a Gengar at level 25, not the other way around.

* Gengar is only available via trading. It's not found in the wild in FireRed.

Therefore, you won't be able to find a Haunter in the wild. You'll need to trade with someone who has a Gengar and level it up to 25 to get a Haunter.