Firewood can be obtained by chopping logs with an ax. One of the jobs in the game involves filling the trailer with firewood and delivering it to the firewood guy. The ax must be equipped in order to pick logs from the log pile. Chopping logs is also a way to relieve stress.
One log gives two firewood. 80 logs will give 160 firewood, enough to fill the trailer completely for delivery. A wood carrier can hold up to 10 pieces of firewood.