In general, most Bow/Crossbow related feats will help, I'll try to focus more on the "must have" for a Firearm user.
MUST HAVE
Rapid Reload, drops the reload time significantly. Pair with Cartridges for extra fun.
Not provoke AoO's just for firing.
Deft Shootist Deed, need Dodge/Mobility ,don't provoke for Reloading either
Point Blank Master, needs Weapon Specialization
Gunslinger (the feat), need Weapon Focus
You regain Grit only if you kill something, or HIT with a Critical shot
Improved Critical (you can't put Keen on a gun)
Critical Focus
Firearms target touch AC most of the time, so the loss in accuracy is negligible, the damage gain is SWEET!
Deadly Aim
If you DON'T plan to take Gunslinger level 7, then this is very nice, otherwise it's too similar to the Dead Shot deed (and slightly worse)
Vital Strike
Make Attacks of Opportunity with your firearms
Snap Shot (Improved/Greater)
The Gunslinger only gets a Bonus feat every 4 levels. Feel free to dip Ranger for a couple levels or Fighter for a lot of levels.
So long as you get Gun Training at Gunslinger 5, you're good.