sounds like you wired it wrong...
the firewall is not a power source, I assume you are referring to the 'fusebox'. if that is the case then that's your problem right there. you are drawing your 12v constant power from a switched power source (and you're drawing from a source that cannot provide as much current as the M4-ATX may require).
the M4-ATX ignition line should be connected to the car's ACC circuit or IGN circuit. it sounds like you already have this connected to an ACC line in your interior fuse box, if that is the case then you can leave it the way it is.
12v constant power should be drawn from the car battery (or an unused slot in the power distribution center; if your car has one), not from the interior fusebox. this will require you to run a [fused] wire directly from the battery to your M4-ATX. the M4-ATX can draw up to 25A under full load. you should run 10awg (or thicker) wire for this.
your ground wire should be the same gauge (or thicker) as your 12v constant wire, and should be as short as possible. run this wire to the closest [chassis] ground point to your carputer.
once you correct your wiring, then it should work just fine. good luck