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Dex is the most important stat for firearms. You won't need much strength unless you're planning on spending a significant time in melee as well (and don't want Weapon Finesse). I'd only recommend that if you've got a generous point buy / rolled array. Your order may or may not require a strong Cha; for example Order of the Lion has two charisma-dependent features but Order of the Dragon doesn't require Cha. Con is always good, and Wis is useful for will saves. Int is optional depending on how much you like skills.
You can use ranged weapons while your mount is taking a double move, but at a –4 penalty on the attack roll. You can use ranged weapons while your mount is running (quadruple speed) at a –8 penalty. In either case, you make the attack roll when your mount has completed half its movement. You can make a full attack with a ranged weapon while your mount is moving. Likewise, you can take move actions normally.
So if your mount only moves its speed or less you can make your attacks, even a full attack, while mounted and at no penalty.
Keep in mind that since musketeer replaces the cavalier's mount you'll need to find a mount somewhere else.