I think the assessment of cedar firewood as the best heat source is misleading and lacking the whole story. It could be considered the best heat source for a traveller, as for a person with limited carrying capacity, burn time to weight ratio is the most important factor. But overall, given that the game's ultimate goal is to last as long as possible, the absolute most important factor is burn time to tool usage ratio, which makes fir firewood - complemented with freely gathered sticks - better than cedar over the long run.