[PvK]

At least for their reaction fire, the order they fire is the order their targets move. I think that's seekers first, then fighters and drones.

For fire during their own turn, assuming they didn't opportunity fire in the previous turn, they target nearest (the first target to come in range as soon as their unit moves a square) and then, I assume, whatever the unit's strategy's settings would have it choose.

PvK