1. Place beef rib rack on a piece of aluminum foil and sprinkle with rib rub. Wrap beef rib rack tightly, transfer to grill, and cook over indirect heat for 4 hours.
2. For the salsa: Roast tomatillo and tomato for 10 minutes or until they turn color. Set aside.
3. In a molcajete, crush garlic and coarse salt. Once a purée is formed, add árbol chiles and slightly crush them. Add tomatillos and tomato and slightly crush them. Add red onion, cilantro, and avocado. Mix thoroughly.
4. Cut beef ribs into single pieces and arrange them in the molcajete. Serve ribs in molcajete with tortillas and nopal salad on the side.