Combine lemon juice and lime juice in glass baking dish. Rinse salmon, pat dry. Place salmon in juices, marinate 10 minutes, turning once.
Combine mayonnaise, mustard and 1 tbsp dill in small bowl.
Preheat broiler. Spray rack of broiler pan with nonstick cooking spray. Remove salmon from juices; discard juices. Pat dry. Place salmon on rack. Broil, 4 inches from heat, 3 to 4 minutes on each side or until salmon flakes in center. Server salmon with sauce. Garnish with fresh dill, if desired.