How is firearm damage calculated?

Question

Is the damage of firearms computed like other ranged weapon attacks? The attacker adds his Dexterity modifier to the damage?

Answer 1

Yes, you add your Dex mod to the damage die

Dungeon Master's Guide presents stat blocks and rules for firearms on pages 267 and 268. They are listed as Martial Ranged Weapons, and no special rules are given regarding damage calculations. Therefore the dexterity modifier is added to the damage just as with any other ranged weapon.

Answer 2

There is a firearm for the UA Artificer's Gunsmith subclass called the Thunder Cannon.

And according to this Sage Advice tweet from Mike Mearls, yes, it adds the Dexterity modifier to the attack roll and the damage, as it is considered a two-handed ranged weapon.