Melt the lard in a large deep saucepan over moderate heat. Add the pork, salt and pepper, and simmer until tender, being careful not to crisp the meat, about 1 hour and 10 minutes. Remove the pork with a slotted spoon and set aside to cool. (Reserve the lard for future use.) When cool enough to handle, shred the meat by hand or with the tines of a fork. Remove and discard any remaining fat. Transfer to a medium saucepan, add the Arbol Salsa and cook over moderate heat, stirring frequently, 5 to 8 minutes. Carnitas can be kept in the refrigerator up to 5 days and reheated before assembling the tacos.

For Carnitas tacos, dip 24 small corn tortillas in water, shaking off excess. Toast one at a time in a nonstick pan over moderate heat about 1 minute per side. Wrap in a towel to keep warm. For each taco, stack 2 tortillas and layer with warm carnitas, Salsa Fresca, chopped cilantro and avocado slices or simply chopped onion and cilantro.