Discard if old or frayed. Exercise care with extension cords. Use no more than three sets of lights per extension cord. Do not place the cords under rugs or in paths of travel. When you take down your tree, do not attempt to burn it in your fireplace.
Dry trees throw off a tremendous amount of heat and and can create a dangerous fire.
Take it to your local recycler to get processed into mulch for your yard. The rest die from inhaling smoke and poisonous gases or from lack of oxygen. Fire consumes oxygen, which feeds a fire. Without oxygen humans and pets will suffocate. Normally, the air we breathe is 21 percent oxygen.