erm.... i think the firebox has 2 inputs with preamp( with phantom power)(1&2), and 2 line inputs (3 &4). remaining 2 inputs (5&6) are via SPDIF. Midi inputs are not counted in the total 6.
which also means that you could plug in
channel 1- mic
channel 2- keyboard
channel 3- guitar (if from amp)
channel 4- bass (if from amp)
channel 5 & 6 via spdif. - have only gotten it to work with my mindprint DI at 44.1khz though..
For your voice it really depends on the mic you use. i strongly suggest using a condenser mic because the preamp(firebox) is
really shit and won;t be able to give enough juice to adequately amp dynamic mics like the shure 57 or 58.. etc..
also one very important thing to consider is your computer setup.
there are known problems on with fireboxes on dells (especially laptops)
and your firewire adapter should be either VIA or Texas Instruments...
you might want to consider the firepod or something else cos at the end of the day- you might want the flexibility of working with more inputs...
but also please note that usually with interfaces- what you pay for is what you get. in terms of stability, quality and reliability. of course you must also weigh that against the costs involved.
btw, i have a love/hate relationship with my firebox.
i hate it but i know its quirks and most of the time, just work with what it is.
i do wish though, that i took the time to save up a little more- do more homework and had gotten something else besides the firebox
just my 2 cents