My first impression is that a D&D Musket or Rifle has a larger diameter than a spear. A spear is the largest diameter item you can fit into the Efficient Quiver (third and longest portion)
I think different GMs will have different ideas though.
Like ngc said, your mileage may vary. I'd probably allow it, perhaps have it count as two spears in the largest section to account for the increased diameter and possibly weight over a spear. But there's no hard and fast rule to point to that says it does or doesn't, so it's ultimately up to your GM.
Probably... Talk to your GM, though.
A musket and most other two-handed firearms (other than the culverin) are pretty comparable in size/shape to a halberd or other polearm.
As a different option, the endless bandolier is cheaper (1,500 gp) and explicitly allows storage of two-handed firearms.