'firebase deploy' Error - Must supply a public directory using "public" in each "hosting" config

Got the same error and fixed it with adding "public" to firebase.json in the empty "public" attribute.

Your firebase.json should be like this.

"hosting": {
  "public": "public", // <- Added
  "ignore": [
    "firebase.json",
    "**/.*",
    "**/node_modules/**"
  ]
}