下記をrewritesに追加する

{  
      "source": "/",  
      "destination": "/index.html"  
}  

全体は下の感じ

{  
  "functions": {  
    "predeploy": [  
      "npm --prefix \"$RESOURCE_DIR\" run build"  
    ]  
  },  
  "hosting": {  
    "public": "public",  
    "rewrites": [{  
      "source": "**",  
      "function": "app"  
    }, {  
      "source": "/",  
      "destination": "/index.html"  
    }],  
    "ignore": [  
      "firebase.json",  
      "**/.*",  
      "**/node_modules/**"  
    ]  
  }  
}  

参考: