From the presonus firebox faq:
Q: What is the difference between input channels 1 and 2 from inputs 3 and 4.
A: Inputs 1 and 2 are Mic/Instrument level inputs for connecting microphones via XLR plugs and guitars via ΒΌ inch plugs. Input channels 1 and 2 have gain knobs located on the front of the FireBox. Input channels 3 and 4 are Line level inputs for connecting devices like keyboards, drum machines or a mixer's left and right outputs.
So you can plug in 1/4 cables:
And xlr cables:
If you go check them out, ask the guy at your local music store and he'll probably hook you up with everything you need (cables, adapters, ... ) to hook up your firebox. Like I said, this is a very decent audio interface and I don't think you can go wrong with something like this.