The society in question is feudal in structure. The most advanced widespread technology defining this society is the flintlock muzzle-loading firearm (I'm undecided as to rifled or not, but leaning towards not). Their capabilities do not extend to mass production of these muskets and pistols, with the vast majority being handmade by craftsmen. The difficulty and expense with which they can be produced is roughly equivalent to the production of knightly accoutrements in the High Middle Ages.
The social structure consists of serfs and bondsmen at the base of the hierarchy. Above them are a landowner/warrior class defined by possessing the wealth to purchase and maintain a firearm (analogous to knights). Above them are the lords, with larger landowning privileges and access to greater armouries, and a feudal king above them.
So basically mid-feudal Europe, but with muskets instead of knights.
The lands this society inhabits are largely analogous to the more difficult areas of northern Europe. The landscape is, by and large, difficult to traverse as it is heavily wooded and mountainous in many areas, making the use of heavy cannon difficult. It's bordered by high mountains on one side, sea on another, and steppe on the last (inhabited by steppe-nomads with similar access to firearms).
Most of Finland might not be a bad analogy, or perhaps areas surrounding the Carpathians.
What military structure and army composition might arise in societies in these circumstances?
I've got a rough idea of small feudal levies armed with hand weapons, acting as anchoring blocks for groups of skirmishing nobles. Use of cavalry might be difficult given the terrain.
Would this be realistic, or are there significant differences to this structure that firearms would change?
If this is too broad/opinion-based please let me know and I'll fix it!