Slavery was already being abolished in the West when Adam Smith wrote the Wealth of Nations. But what was notable was that Adam Smith was really the first to make a strong case and prediction that it was not just the moral thing to do, but would lead to prosperity.

Adam Smith also differentiated between different levels of slavery - that Roman slavery was different than Serfdom was different from chattel slavery in the US.

It's worth noting that Adam Smith did not think total abolition was possible. One of his concerns about free markets was that people deeply desired control of other people, and slavery would increase as a byproduct of wealth.