The moisture content of fire wood influences the heat efficiency of a fire. The higher the moisture content of the wood, the less heat efficient a fire becomes.
Moisture content also plays a role in the formation of slag in the flue system. The higher the moisture content of the wood, the faster slag build up would take place in the flue system.
Always make sure that the woods you use are from a sustainable source. Only wood used from a sustainable source could be considered as carbon neutral. Were possible utilise alien species fire wood in support of programs like the working for water program, a sustainable initiative creating employment to the poor and having positive impacts on the environment.