Would nicely fit in Eco game. Firewood could also be a byproduct of a lot of woodcutting crafts (I personally use those sawmill byproducts to heat my house), incentizing you to sell it. For burning efficiency with wetness, we could use the same system as food spoil : the firewood have a full wetness bar when produced, then it decays when stored in an outside "firewood rack" overtime. When the bar is empty, the burning efficiency is maximum (more joules per item, about 100% than with greenwood). To split logs to firewood, you could use an axe on the ground, or a sawbuck workbench (for everyone to use, made by woodcutters and they use lower calories), or the sawmill later on (less calories, same firewood amount).
Great Idea. (even just an initial approach just splitting logs to firewood) ie: 1 log : 4 firewood ... and instead of having 100% J of power, having 50% J ... so *4 would give a total of 200% of a normal log :) Halfway between log and coal <3 ---> Possibly even done in a normal workbench.
I'll just add onto this by having the carpenter being able to recycle basically anything wooden into firewood if it isn't directly burnable already. And yes, firewood byproduct from hewing would be fascinating, in addition to splitting logs/hewn logs for fuel. Heck, I'm already doing that with boards.