From using my Firepod, channels 3-8 don't have the Hi-Z inputs so if you're using a microphone and plugging it into those channels (3-8) with a 1/4" plug, the preamps don't work for that. Those channels are for plugging in line level sources like a keyboard or another preamp. The first two preamps have enough gain and are designed for that, but the last 6 aren't.
I'm pretty sure that is the correct way of describing this. Just use XLR cables with your mics and you'll be fine.
If I'm wrong on this, somebody please enlighten us.