First things first, let's get the Firebase CLI tool installed. If you haven't done this yet, you can easily do it with npm:

npm install -g firebase-tools

Now, head over to your project directory in the terminal and run:

firebase init

When prompted, select "Hosting" and follow the steps. This will create a firebase.json configuration file in your project directory.

Open up the firebase.json file. To support multiple sites, you'll need to use the hosting array configuration. Here's an example setup:

{
  "hosting": [
    {
      "target": "site1",
      "public": "site1-public",
      "ignore": [
        "firebase.json",
        "**/.*",
        "**/node_modules/**"
      ],
      "rewrites": [ ... ],
      "redirects": [ ... ],
      "headers": [ ... ]
    },
    {
      "target": "site2",
      "public": "site2-public",
      "ignore": [
        "firebase.json",
        "**/.*",
        "**/node_modules/**"
      ],
      "rewrites": [ ... ],
      "redirects": [ ... ],
      "headers": [ ... ]
    }
  ]
}

In this example, site1 and site2 are the IDs for the different sites. The public field specifies the directory from which files will be served.

Next, find your Firebase project IDs for the different sites and set them using the Firebase CLI:

firebase target:apply hosting site1 <projectID-1>
firebase target:apply hosting site2 <projectID-2>

Replace <projectID-1> and <projectID-2> with your actual Firebase project IDs.

To deploy the sites, use these CLI commands:

firebase deploy --only hosting:site1
firebase deploy --only hosting:site2

Or, if you want to deploy both sites at once, just run:

firebase deploy

If you need to, you can customize the rewrites, redirects, and headers sections within each hosting configuration in firebase.json. Each site can have its own specific rules:

"rewrites": [
  {
    "source": "**",
    "destination": "/index.html"
  }
],
"redirects": [
  {
    "source": "/old-path",
    "destination": "/new-path",
    "type": 301
  }
],
"headers": [
  {
    "source": "/path/to/file",
    "headers": [
      {
        "key": "Cache-Control",
        "value": "max-age=3600"
      }
    ]
  }
]

Tweak these as needed based on what each site requires.

This setup lets you host multiple sites using a single Firebase project, taking advantage of the flexibility of the firebase.json configuration.