Here in Sweden at least, christmas trees are not pines but spruce.
These small ones with lots of branches - probably not worth the work effort unless you are desperate. And you'll get a lot of residue branches. You also want them to dry for a season before you burn them.
Other than that, they burn just like any other log. Maybe a bit less energy per log. Maybe more smokey than some, but smoke should go out the chimney not in the room.
It's worse than that. The sap in it will collect in the chimney and then cause a chimney fire. You really don't want to burn that stuff in a fireplace.
It's both too low temperature burning leaving resiude in the chimney that can catch fire later, AND pine/spruce having way too much sap to burn alone. You should absolutley mix it with other wood when you burn it, same as oak.
These small ones with lots of branches - probably not worth the work effort unless you are desperate. And you'll get a lot of residue branches. You also want them to dry for a season before you burn them.
Other than that, they burn just like any other log. Maybe a bit less energy per log. Maybe more smokey than some, but smoke should go out the chimney not in the room.