firebase.json

{
  "hosting": [
    {
      "target": "dev",
      "public": "heroku/static",
      "ignore": ["/*.conf.json"],
      "rewrites": [
        {
          "source": "/conf.json",
          "destination": "/dev.conf.json"
        },
        {
          "source": "**",
          "destination": "/index.html"
        }
      ]
    },
    {
      "target": "staging",
      "public": "heroku/static",
      "ignore": ["/*.conf.json"],
      "rewrites": [
        {
          "source": "/conf.json",
          "destination": "/staging.conf.json"
        },
        {
          "source": "**",
          "destination": "/index.html"
        }
      ]
    },
    {
      "target": "production",
      "public": "heroku/static",
      "ignore": ["/*.conf.json"],
      "rewrites": [
        {
          "source": "/conf.json",
          "destination": "/production.conf.json"
        },
        {
          "source": "**",
          "destination": "/index.html"
        }
      ]
    }
  ]
}