Let's do a rundown of the classes. I'm going to go with non-magic heavy classes (so no medium casters or higher): Barbarian, Cavalier/Samurai, Fighter, Monk, Paladin, Ranger, and Rogue/Ninja.
BARBARIAN
Defense: The class already assumed lesser armor, so the benefits built in to offset this (extra movement, DR, some rage powers), help with this. Some rage powers help specifically (Rolling Dodge for one).
Offense: While the standad rage doesn't really help, the Urban Barbarian's "Controlled Rage" variant makes for a nice alternative: no AC penalties and can apply the bonus to Dex. Between that, and the Surprise Accuracy rage power tree, he's got at least a few things nice to apply to firearms.
Notes on Controlled Rage: In a more advanced technological game, this makes more sense as the standard "rage" for such an era. The Str/Con rage would be good for more savage/tribal types, but a 'Barbarian' in the old west, someone who controls his adrenaline rush to gain specific focus, would work better with the Urban version. Simply swapping the rages (or making the Urban Barbarian the default) would solve your problem here.
For an example of an extreme version of such a fictional character, check this youtube link (violence, swearing).
[ame="http://www.youtube.com/watch?v=y_JGI0JhCkQ"]Equilibrium - Not without incident - YouTube[/ame]
Final Thoughts: The Barbarian is pretty well off, if you keep in mind that the game has switched from "primarily melee" to "primarily ranged" combat.
Simply having a lot of speed and the pounce rage power can make you a powerful force, even in a world of guns. With the archetypes and alternative rage powers, you should even still be able to do very well.
CAVALIER
Defense: Not much from the class directly, although having a mount can mean getting into close range is easier (good movement despite armor). High hitpoints help as well.
Offense: Teamwork feats like Enfilading Fire or Target of Opportunity (and the fact that he can give them to others temporarily), are quite nice. The Strategist archetype just amplifies this factor, while the Musketeer gives a workable "kinda-gunslinger" type class.
Final Thoughts: Not huge on defending against firearms. Strong offense as your good defense might be the way to go with this one.
FIGHTER
Defense: While armor might not be useful, they can retain high Dex to AC despite the armor, meaning more versatile against varied foes.
Archetypes can help a lot too. Armor Master gains touch AC and DR, and Tower Shield Specialist gains touch AC down the line too.
Offense: Ranged fighter builds work. They can gain tons of feats, and fast, so not only do they get a lot, but they will have long trees filled sooner. Amateur Gunslinger means they can do pretty decent without actually being a gunslinger too. They have weapon training too.
Final Thoughts: The fighter is probably as well set up for firearms as he is for other ranged weapons (which is good). The touch AC options are available too.
MONK
Defense: High movement, high touch AC, deflect arrows applying towards bullets. Yeah.. he's not in a bad way, and you don't even have to get into archetypes that might enhance this even better.
Offense: Being Dex focused works fine with this class (in fact, helps against MAD), fast movement means you can get to your enemy fast and start pummeling him. A tweaked Zen Archer tweaked to work with Firearms (Gun Kata! Whoo!) would make them amazing with firearms.
Final Thoughts: The Monk is probably in the same boat as the Barbarian. Decent defense (especially with deflect arrows feat), and with a minor tweak he can be pretty good with firearms too (Zen Archer).
PALADIN
Defense: Not much in the base class itself, other than self-healing. Even the archetypes don't really go into much. High hitpoints are nice.
Offense: Holy Gun and Divine Hunter are two good archetypes to focus on the firearm or ranged weapons in general. Smite adding to damage and bypassing DR can help with making firearms stronger.
Final Thoughts: The Paladin is about the same as the Cavalier.. okay, but nothing amazingly stellar.
RANGER
Defense: Assumed lighter armor, and potential defensive benefits (terrains and favored enemies with certain feats).
Offense: Well, there's ranged combat styles, and favored enemy benefits. There's the Trophy Hunter archetype for firearm specific benefits.
Other archetypes (gaining rage powers, or other effects) cross enhance the class.
Final Thoughts: Being a ranged weapon class to begin with (or at least a primary option) easily keeps this class competitive with firearms. Specific archetypes help as well.
ROGUE
Defense: Easy to Dex focus allows for decent touch AC. While talents aren't amazing, there's a few that might help survival.
Offense: Sneak Attack. Sniper Goggles. Play the firearm rogue like a ranged combat rogue. There's a talent for gaining proficiency too.
Final Thoughts: Honestly, Sneak Attack combined with Touch AC makes for a deadly sniper combination...
OTHER OPTIONS
Amateur Gunslinger: This is a feat anyone can take. Gaining proficiency is no longer a pain, since it's a simple weapon, it's a good way to make anyone a little bit like a gunslinger.
Bullet-Proof Armor (homebrew thoughts): While they neglected to add any rules for this concept, it's understandable since the defacto standard is emerging guns era.
However, in an era where guns are so commonplace, very likely armor has been advanced to handle the power of a firearm. I'd probably run it like the mechanics for composite bows work with adding strength bonus.
Maybe something like +500gp per 1 AC that counts towards firearm attacks, up to the max AC of the armor, metal armors only (excluding chain?).
Overall, I think the classes are mostly okay, with maybe a minor tweak here or there (Controlled Rage, Zen Archer). Adding a type of Bullet-Proof armor would make higher levels (where it's affordable) better for the defense factors of high armor classes (like the Cavalier and Paladin).
That's about the only changes I'd really consider for such a campaign.
