What happens if the beads from a necklace of fireballs are used as ammunition for a firearm? Would loading count as throwing so that when the bead stops in the firearm's barrel, the bead explodes? If not, should the bead make a Reflex saving throw (DC I don't know) to avoid detonation? If a bead were fired, should the bead detonate on impact without dealing normal firearm damage, or should the bead deal normal damage, stop in the struck target's body, and detonate there, eliminating the target's Reflex saving throw?
EDIT: Answer completely re-worked on June 5; some comments may no longer apply.
This comes down to two issues.
What actually causes the detonation? The magic item says the beads detonate when exposed to magical fire or "when thrown". What is it about being thrown that allows them to detonate? Most likely the impact is the trigger that causes detonation once the beads are armed. The question is, what arms the beads?
Do they have to be primed somehow (like pulling the pin from a grenade), and if so how is that done (is simply detaching it from the necklace enough)? Do the beads somehow telepathically read the carrier's intent, so that they can distinguish being deliberately thrown from being accidentally dropped? Are they activated by reaching a certain velocity? Or are they always armed, and any impact above a certain threshold causes them to detonate?
If it's always armed or has to be primed before it's rammed down the barrel, then the blast from the black powder will almost certainly set it off (thanks Pulsehead for pointing out that it could be carefully rammed down the barrel without setting it off). If it's activated by the user's intent, then it can't be used with a firearm at all, since it won't detect that it's being thrown.
If, however, it's armed by reaching a certain velocity then triggered by impact, then you can probably use it in a firearm without any real risk of it detonating before it reaches a target after being fired.
As for once it's fired, if it strikes a target it should probably detonate immediately; the beads are intended to explode just from being thrown, so the impact of striking a target at firearm velocities should set it off instantly, rather than giving it time to penetrate. If it explodes from colliding with the target, there's not much the target can do to avoid the blast, so the target shouldn't receive a reflex save. Since the bead is being fired from a weapon, however, the attacker should have to make a typical attack with the firearm to actually strike the target.
Does the DM want magic and technology to be combined like this? While it can allow a lot of interesting creativity (such as firing fireball beads from a musket), it also probably opens the door to a lot of potentially game-breaking combos. Magic and technology together are likely to be a "the whole is greater than the sum of the parts" kind of thing. You might even take the route used in the Amethyst setting and say that proximity to magic actually prevents technology from working.
Crossing combustion and gunpowder with fire-based magical artifacts is tricky. If the concept was to sling the beads with a slingshot, it would be easier, but since you want to blast them of with igniting gunpowder, I would as a GM rule that the chance to activate them inside the gun-barrel would be too great.
Since firearms are already a rather odd bird in most fantasy settings, I think an interesting twist would be, rather than using muskets as just a fireball delivery mechanisms, to have these technological artifacts actively resist magic. Integrate their alienness into the system.
I'm drawing much of this from Arcanum, the excellent old CRPG, where characters could advance as technologists or as magic-users, but not both - either one will cause the other to malfunction. Similarly, attempting to fire a magical projectile will cause the musket to misfire, while attempting to enchant one will cause the spell to rebound, hopefully in interesting and creative ways.
Of course, that's just my suggestion. You might like the mix of gunpowder and magic. It's just that I feel that edging closer to post-medieval technology tends to take a lot of the oomph out of magic in D&D/PF campaigns. I never liked AD&D2e's arquebuses, either. :)