It's a fissile element in Nuclear Reactors, not anything that you can physically burn. The Lore indicates it is used as such, and the reactors power some sort of plasma thrusters or nuclear rocket. Presumably an MOX based reactor system would simply be the power plant for a similar technology (or extrapolation thereof).
As a fuel or fissile element, MOX is highly impure though. It's cheap for electricity generation, but for anything really cool or explosive you'd need highly enriched, weapons grade uranium or plutonium, and that is presumably tightly regulated. Don't ask me how I know this.
Presumably, H-fuel is used for smaller craft, and MOX in the reactors for larger ones. Nuclear fission is a well understood process, but fusion is a bit trickier, since we've yet to get out more energy from a fusion reaction then we put in as of the 21st century. We could extrapolate it that fusion engines are somehow haxy or impractical for spacecraft propulsion technologies in the FL verse.
For Vanilla FL's developers to understand that something as obscure as MOX would be used in a way sort of hints at that.
Then again, Freelancer tends not to stick too tightly to real world physics.
