Seems like a great way to stock up your firewood supply for the next winter, if you manage to target it in a way that doesn't cause half the city to show up, but maybe a little less than that?
We lit up a dead Christmas tree that had been sitting outside one summer. We were so sure the cops would show up when someone saw the two story column of fire that thing made. The good part is that it was all over in about a minute.
How dry are we talking? Because burning furniture-grade EWP offcuts (~6% moisture content) is smoky as hell. It isn't the wood, at least not entire - it's the sap pockets and other crap.
(I have a wood shop, these are things relevant to my interests!)
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.