Whisk together 4 eggs, 4 yolks (reserve remaining whites), and 1 cup sugar in a 4 qt saucepan. Add salt and juice and zest; stir until smooth.

Place saucepan over medium heat; cook, stirring, until mixture thickens to the consistency of loose pudding, about 10 minutes. Pour through a fine strainer into a large bowl and chill.

Whisk egg whites and remains sugar in a bowl until stiff peaks form; add to curd. Fold gently until combined; set aside.

Whisk cream and vanilla in a bowl until stiff peaks form; add to curd mixture. Fold until combined. Spoon mousse into serving cups; chill before serving.