You don't react to a FFE if you want to be aggresive. What's important is what he does after that. If he begins teching, throwing down more gates, etc.
If you try to be aggresive just because he went FFE you run the risk of just straight up dying if he picks an aggresive build.
There are a couple different builds, but most of them depend on what he's doing, if he sees them coming, and how early you want to hit. What really matters is if he does FFE> nexus>cannon> makes no untis until x time (because protoss are greedy)/follow build orders rigidly.
But overall, aggresive responses are "worse" than just taking a third hatch.