To even be considered, it would have to be limited to firearms, rather than the Exotic Feat itself.
I think game balance in this instance (and most of the time) should trump "realism" considerations, and even in terms of realism, we live in a gun-present society in the US without the average person being proficient in their use. To be sure, they are easier to become relatively skilled with than with a rapier, say. Yet the chance of picking up a gun and hurting yourself or someone else by accident is much higher, too. Although it's possible to misclick and shoot an ally directly in POTM, without any training I doubt very much a Port citizen could pick up a firearm and risk shooting *through* friends into enemies (not to mention that misses continue on IRL until they hit something). In other words, it may be realistic that anyone can equip a firearm, but firearm use in POTM is easier and not realistic in other ways. (I don't think 1600s firearms are as easy to equip and use as their modern equivalents.)
So back to game balance, are firearms under-powered and not worth a feat investment? Then make them not exotic for everyone.
Are Port characters under-powered? How? Presumably they don't have to invest the encumbrance armor requires most PCs, nor do they then have to enchant it. (If Port PCs are wearing armor and enchanting it, then the argument that they should gain firearm proficiency is undermined because they are not RPing as someone who "naturally" lives in a firearm culture.)
The argument that even citizens in the United States aren't universally proficient with firearms falls apart when you consider that, yes, maybe Commoners wouldn't? But anyone with any class that has Martial Weapons Proficiency would. I would also ask why every living Dwarf is proficient with a Dwarven Waraxe, which is exotic to all but Dwarves, and why Elves are all proficient with Longswords, Rapiers and Longbows (the latter of which, is actually far more difficult to learn to use than a firearm) and so on, and so forth.
I don't neccessarily believe that providing WF: Firearms would make Dementlieuse characters any more Overpowered than Dwarves, or Elves. In-fact, I somewhat gain the impression that if Dwarves and Elves hadn't already received those proficiencies, and we weren't already used to the idea that they received them, there would be an intense argument about why giving them those proficiencies would upset the balance of the game.
I could see it now; Elves have Keen Senses, Hardiness vs. Enchantsmens, +2 Bonus to Listen, Search and Spot checks, and Low-light vision, +2 DEX, -2 CON, access to Rapier, Longbow, Longsword, and Shortbow proficiencies all those things for FREE. The cheek of it!
And Dwarves? Oh, God, don't even get me started. They're the unsung heroes of NWN.
They get +2 Search underground, Darkvision which is better than low-light vision, +2 vs. Poisons, +2 vs all saving throws against spells, +1 Attack bonus vs. Orcs, and Goblinoids, +4 AC bonus vs Giant, and +2 Lore check. They get +2 CON which is a major buff more-or-less any class in the game, and only a -2 Charisma penalty, which is useless for anything but a handful of casting classes!
And
then they want to get access to a 1d10 x3 one-handed Battleaxe for free? The cheek of it! If they're playing as a martial class, they can afford the exotic proficiency. Unheard of.
And what do
HUMANS get? Standard stats, a bonus feat, and 4 extra skill points? Really, everyone could just have that, and re-invest it as they please.
There's my satire, for the day. I thought it would be thematically appropriate for Dementlieuse to have better access to them, even if it would cost them somewhere else. Because, what self-respecting Dementlieuse wouldn't know how to shoot a gun before they learned the complexities of swordplay, to use a bow, or to cast a spell? After-all, those are all things which are infinitely more complicated to achieve or accomplish, and learning how to use a firearm can be done in a fraction of that time. Thus why no-one fought in the melee except for bayonet charges after basically the late 1600's, after the flintlock mechanism became popular. The time period Dementlieu emulates.