I suggest hardwoods only because if you are going to use it to cook with, then you don't want to use fir. Most coniferous woods contain too much of the nasties that would taint whatever you are cooking something fierce. About the only thing I will cook over an open fire made from fir is hot dogs or marshmallows, where I can keep the food out of the path of the off gases from the combustion.

However, for your purposes, I don't see any fault with using fir. Also, it depends on how big a dutch oven you use. : ) I have or can get my hands on some pretty big ones, big enough for what you need for sure.