Firearms in The Division is a Stat that increases the effectiveness of the player with Weapons.
Improving this Stat will increase your overall damage with Weapons. Those wishing to improve their DPS will want to find and equip gear with this Stat bonus. Your final DPS stat is a combination of the weapon's base DPS plus the DPS that is calculated from your Firearms stat.
Although The Division does not have a class system, choosing to focus on one of the Stats is a method of specializing a role. The Firearms stat is ideal for the player who wants to develop a DPS or damage dealing class at the expense of survivalbility i.e. glass cannons.