I'm playing D&D 5e, and I'm not sure what ability score applies to firearm attack rolls. (Using the stats from the DM's Guide.)
Every Firearm in the DMG (p. 268) is a Ranged weapon. And the rules state (PHB, p. 194)
the ability modifier used for a ranged weapon attack is Dexterity.
If the creature using the firearm is proficient with them (up to the DM to determine), then they would also add their proficiency bonus to the roll to hit (but not the roll to damage, which would just add the Dexterity modifier).