The law of conservation of mass states that mass cannot be created or destroyed in a closed system during a chemical reaction. In the case of burning firewood, you're observing the transformation of wood (which is mostly composed of carbon, hydrogen, oxygen, and other elements) into ash, gases, and heat.
When the wood burns, it reacts with oxygen in the air, producing carbon dioxide, water vapor, and other gases, along with the solid residue of ash. The reason the mass of the ash is less than the original mass of the wood is because a significant portion of the mass has been transformed into gaseous products and released into the atmosphere during the combustion process.
If you consider the entire system — the wood, the oxygen consumed from the air, the produce gases, and the ash — the total mass of that system remains constant. However, if you only account for the mass of the remaining ash after burning and ignore the gases that have been released, it might appear that mass was lost.
In summary, while the mass of the remaining ash is less than the initial mass of the wood, the law of conservation of mass still applies when considering all components of the system involved in the reaction.