Why is firewood essential for making salt? In the ocean water, 35 parts per thousand are sodium chloride (NaCl), with the NA+ and Cl- ions in solution. To concentrate that, Virginians would boil the salt water in kettles, driving off the water as steam and precipitating a crust of mineralized salt on the kettle. It took a lot of firewood to boil down the water kettles - and a lot of patience too. On a hot summer day, making salt wasn't a fun chore.
During the Revolutionary War, the British blocked most of the trade in salt between the Caribbean islands and Virginia. The same occurred in the Civil War. Coastal plain residents could cope with this hardship, especially by using slave labor to make salt. Across the Blue Ridge, however, salt was always expensive and occasionally hard to acquire at any price. It had to be hauled upstream, across the mountains on roads barely fit for wagons even at the time of the Civil War. (Watch an August thunderstorm, the imagine travelling the dirt roads for the next day or two...)
A few natural salt licks in the western part of the state were highly prized. "Big Lick" was an animal concentration point before it became Roanoke - and mastodons concentrated at salty marshes that later were the site of Saltville.