I think a good place to start would be Node Express. With Node Express you can get setup quickly, and can create routes in your API that call Firebase Cloud Functions directly. Express is also very flexible so you have a lot of freedom to setup the API and configuration the way you want to. Checkout the Express site here. I was also just looking at the Firebase Cloud Functions Documentation, and saw where they had instructions specifically for Express here.
One other note, the AngularFire library provides ways to directly call Firebase Services. So with AngularFire you don’t necessarily need to create a full orchestration layer in your project. It all depends on your project needs, etc. Hope that helps. Thanks for the question!