Not much is known about Firearm and unlike the other members of his team the identity of the man underneath the armor has never been revealed. We do know he is a black man who was once a Guardsman at the Vault. General Orwell Taylor recruited Firearm and a number of other men into the Jury to help the avenge the death of his son, Hugh. All the men personally liked Hugh and in the beginning, were willing to fully work with Taylor to avenge his death. As a guard at the Vault, Hugh was killed by Venom during an escape attempt, though Venom regretted the need for this death.
In one of their first missions, they track Spiderman with an array of sensing devices on rooftop property; all legal. After delibratly taunting the devices, Spiderman is surrounded and defeated. Most of the Jury are willing to kill Spiderman then and there but they listen to the General's orders in the end. He is drugged and put on trial, for the crime of bringing Venom to earth.
Screech once comments that Firearm had joined the Jury in order to perform an autopsy on Venom's alien symbiote, indicating scientific leanings.
Due to an art error in Thunderbolts #23, Firearm is shown to be a blonde white man. Presumably Kurt Busiek or Mark Bagley confused Firearm with former member Screech (Max Taylor) who had by this time given up his armor and was no longer an active member of the team.
Firarm and the Jury were later led by the U.S.Agent and financed by Edwin Cord. They were sent against the Thunderbolts but failed. Later the Jury would team up with the Thunderbolts against the new Secret Empire.