Final answer:
Parachutists descending from disabled aircraft are protected by international law and cannot be targeted, while combatant parachutists participating in an assault can be legally fired upon.
Step-by-step explanation:
The question of whether parachutists are considered combatants and can be fired upon while they are still in the air tends to fall under the purview of the laws of armed conflict or international humanitarian law. Specifically, the Geneva Conventions and their additional protocols provide regulations on the treatment of paratroopers in combat. According to these laws, parachutists who are descending from an aircraft that has been disabled are protected and cannot be targeted while in the air. However, combatant parachutists participating in an airborne assault do not have this protection and can legally be fired upon by the opposing force. This distinction is critical in ensuring the humane treatment of personnel in times of war.