Muskets are used by all the factions, but usually seen only among infantry's riflemen formations - musketeers and among the navy crewmembers.
A lot of times musketeers will also use a special military "fork" to help them position their weapon, it's a sort of a long wooden stick which is impaled in the ground and then serves the same purpose as a tripod for modern photo cameras :)
It's not mandatory to have these forks, but muskets are quite heavy, so it's easier to fire them this way for their users.
It's also easier to target enemies with a fork, than without it.
After shots are fired, if the enemy is close, they might also be used as a blunt weapon, a club of sorts.
