You don't need a DI as your firepod already has them as others have stated. However, a nice DI like a Radial J48 will probably get you a slightly better signal to work with - perhaps even noticeably. I haven't tested this myself with a firepod but I know that my Radial J48 slays my POD XT Live's DI over USB mode, and it also slays the Guitar Rig 2 DI, in addition to the instrument DI's on my MOTU 828 mk ii interface.