according to some, firing up is slightly louder depending on the sub and application, I have a downfiring box right now, it sounds great. Just make sure there is clearance for excursion if it's downfiring.
It makes no difference in terms of performance of the sub. However, the direction the sub is facing will make a difference relative to the acoustics of the vehicle. Depending on the vehicle it could be louder firing up, down, or back.
depends on the vehicle and sub.I got a friend who has a durango W 3rd row seat,and 2 diamond audio d1 12s.they are in a prefab box that has a little port noise and subs tend to bottom out being overpowered on a kx600.1.so he made some little 1 .75 inch blocks and flipped the box over(downfiring)and it sounds much better,cleaner and deeper.the subs will go eventually but for now the downfiring is a quick fix.you cant here the bottoming or port noise anymore(but its still there).