You can first use rubbing alcohol to clean soot off the inside of the jar, before you begin cleaning out the wax. Also, it will help remove any stubborn spots from the label on the outside of the jar when you peel it off.
As simple as it says, but the slower of the two methods.
Place the jar in the freezer, and wait a few days. Eventually the wax will shrink away from the sides of the jar, then you can carefully use a butter knife to scrape it from the sides.
This method requires the use of heat, so please USE CAUTION and DO NOT burn yourself.
Set the jar in a sauce pan with water. Then, turn up the heat to around medium until the water is boiling lightly and the wax on the inside melts enough to pour out the jar, using a pot holder when you pour.
Run your water in the sink as hot as you can without burning yourself, obviously.
This will loosen up the remaining wax. Pour out the water and with a paper towel, wipe and buff out the remaining wax. Repeat this as often as needed until all the wax is gone.