You should not feel guilty for using nature's readily available heating source! For thousands of years, mankind has utilised the energy captured within the tree for heating and cooking. It is only relatively recent times that we have ventured into using electricity, gas, oil to meet our heating needs. However, these sources come with their own price tag and can be costly on the wallet and on the environment.
The Firewood Association of Australia has this to say comparing the environmental benefits of using firewood for heating.
"Firewood is not often recognised as "green" energy, probably because it comes from trees. But, unlike other sources of "green" energy, firewood does not need major capital investment or infrastructure. Every tonne of dry firewood contains approximately 20,000 Megajoules of stored energy.
To understand how this stored energy compares to other forms of "green" energy, such as wind power generation, you need to convert the electricity produced from Megawatt hours into Megajoules. One Megawatt Hour equals 3,600 Megajoules.
This means that a typical 660 kW wind turbine will generate around the same amount of energy in one year as that contained in 315 tonnes of firewood. Recent estimates (Driscoll et al 2000) place Australia's annual firewood consumption at around 4,000,000 tonnes. This is equivalent to the annual output of nearly 13,000 wind turbines or six large (550 MW) coal fired power stations."
(Excerpt from http://www.firewood.asn.au/the-facts.html)