Bring a large pot of water with 2 tablespoons salt and ¼ cup (60 ml) lime juice to a low boil over medium heat. Working in batches of about 6, poach the shrimp until they are firm, opaque, and have a ‘C’ shape but are not tightly curled. The internal temperature should read 145˚F (63˚C). Transfer the cooked shrimp to the refrigerator to cool completely.
In a medium bowl, mix together the mayonnaise, remaining 2 tablespoons lime juice, Fiery spice blend, Dijon, shallot, jalapeño, and chives.
Toss cooled shrimp in the dressing to coat, then fold in the cucumber, mango, and avocado. Season with the remaining teaspoon of salt and the pepper.
Serve the shrimp salad in the lettuce leaves. Garnish with chives, mango slices, and more Fiery spice blend, if desired. Serve cold.