Slice off top of each tomato; scoop out pulp, leaving shells intact. Reserve pulp. Sprinkle inside of each tomato shell with 1/8 tsp. salt; invert on paper towels.
Chop reserved pulp. Peel, seed, and mash avocados; combine with chopped pulp and next 3 ingredients. Stir well. Fill tomato shells with avocado mixture; sprinkle with bacon. Serve tomatoes on lettuce leaves.