data.json

{
  "game": {
    "1": {
      "players": {
        "a": true,
        "b": true
      }
    }
  },

  "users": {
    "a": {
      "name": "Daniel"
    },
    "b": {
      "name": "Simen"
    },
    "c": {
      "name": "Seb"
    }
  }
}

mapFirebaseToProps.js

// In theory:
const mapFirebaseToProps = props => ({
  users: {
    'a': '/users/a',
    'b': '/users/b'
  }
})

// In practice:
const mapFirebaseToProps = props => ({
  players: mapValues(props.players, (value, key) => `users/${key}`)
})