404 on directly hitting page url. browser build

Ionic Framework
4 
"hosting": {
    "public": "www",
    "ignore": [
      "firebase.json",
      "**/.*",
      "**/node_modules/**"
    ],
    "rewrites": [
      {
        "source": "**",
        "destination": "/index.html"
      }
    ]
  },

This is mine.
See the difference in destination? The destination directory is local to the server and the assets are with your rules taken from the local PC folder www, but not placed by default also in a www folder on the server. Basically at firebase init time (or manual changes) you have incorrectly added www to the parameters for the rewrite destination. Maybe try with the www removed?