1. In a large glass baking dish, place chicken in a single layer. Mix lime juice, oil and salt and pour over chicken, turning several times to coat.
2. In a medium-sized serving bowl, combine avocados, tomato, onion and cilantro. Mix thoroughly and set aside.
3. Place chicken breasts on grill and cook about 3-4 minutes per side until they start to brown and are no longer pink in the middle. Remove from heat and top with avocado salsa.