Grounding
is best done like Bill suggests, but common sense needs apply when you evaluate your particular situation. Battery forward? Then ground to the Battery. If Battery aft there is nothing wrong with using the airframe for ground return to avoid running another heavy wire.
The FOT can be directly grounded to the firewall. No matter where you ground the FOT you avoid ground loops by using it exclusively, when possible, for grounding, which keeps the potential between things under control.