In a mixing bowl, whisk together lime juice, olive oil, cumin, chili powder, garlic powder, salt, and black pepper.
Place the chicken breasts in a large resealable bag and pour the marinade over them. Seal the bag and marinate in the refrigerator for at least 15 minutes, or up to 2 hours.
While the chicken is marinating, prepare the avocado salsa. In a separate bowl, combine the diced avocados, tomato, red onion, cilantro, and lime juice. Gently mix and season with salt and pepper to taste. Set aside.
Preheat the grill or grill pan over medium-high heat. Remove the chicken from the marinade and discard the marinade.
Grill the chicken for about 6-7 minutes on each side, or until the internal temperature reaches 165°F (75°C) and the chicken is cooked through.
Once cooked, remove the chicken from the grill and let it rest for a few minutes before slicing.
Serve the grilled chicken topped with the avocado salsa.