They did practice slavery.

There was a definite hierarchy of families in Liberia, with the first 300 families or so forming the topmost level of society, which more or less socially emulated the antebellum South "complete with top hats and morning coats and masonic lodges. They built houses with pillared porches, gabled roofs, and dormer windows resembling the nineteenth-century architectural styles of Georgia, Maryland, and the Carolinas." (Martin Meredith. The Fate Of Africa: A History of the Continent Since Independence. New York: Public Affairs, 2011. Page 545).

Below them, came any of the various other former-slave families that came to Liberia after emancipation. And lowest of them, were the native Liberians, though some of the natives did manage to, marginally, improve their position by becoming "country boys" or "hinterlanders" who were able to improve their social standing by being recognized and patronized by one of the Americo-Liberian families. It was these "uplifted" natives that supplied the vast majority of the bureaucratic and clerical workforce for the Liberian government. All the others were mostly ignored or utterly exploited.

To again quote Meredith: "Just like the white settlers in Africa, the Americo-Liberians constructed a colonial system subjugating the indigenous population to rigid control and concentrating wealth and privilege in their own hands. Despite their origins as descendants of slaves from the Deep South, they regarded black Liberians as an inferior race, fit only for exploitation." (pp 545-46) And yes, this did involve slaves. "In 1931...an international commission found senior government officials guilty of involvement in organized slavery." (p 546).