In a small saucepan, over medium-high heat, whisk together the orange juice, sugar, mustard, 1/4 cup of the vinegar, and 1/4 teaspoon of the salt. Bring to a boil, reduce heat to medium-low, and simmer, uncovered, for about 8 minutes. Season with 1/4 teaspoon of the pepper.
Heat broiler. Arrange the salmon on a foil-lined rimmed baking sheet. Brush the salmon with the glaze. Broil until the salmon is the same color throughout, about 5 minutes, basting once with the remaining glaze. Place the cucumber, lettuce, mint, and orange in a large bowl. Drizzle with the remaining 2 teaspoons of vinegar, the oil, and the remaining salt and pepper; toss.
Tip: Use the tender inner leaves from the Bibb lettuce, not the outer tough leaves. Your reward will be a pleasing buttery taste and a supple texture.