Alright, so first things first, you need to locate the firebase.json file in your Firebase project directory on your computer. Once you find it, open it up with your favorite text editor. Notepad or Visual Studio Code works great for this.

Next, you want to find or create the "hosting" section in this JSON file. If you don't see a hosting section, no worries! You can add it yourself right after the { like this:

"hosting": {
}

Now, let's get into the fun part—adding headers. Inside the hosting configuration, you'll need to define a "headers" section. It should look something like this:

"hosting": {
  "headers": [
  ]
}

For each custom header you want to add, you'll need to create a new object in the "headers" array. Each header object should have two properties: "source" and "headers". The "source" property is a glob pattern that specifies which paths should have this header, and the "headers" property is an array of header objects. Each header object should have two properties: "key" and "value".

Here's an example of a header configuration that adds a Content-Security-Policy to every path ("source": "/**").

"hosting": {
  "headers": [
    {
      "source": "/**",
      "headers": [
        {
          "key": "Content-Security-Policy",
          "value": "default-src 'self' https://*.firebaseapp.com"
        }
      ]
    }
  ]
}

Once you've added all the headers you need to your firebase.json file, go ahead and save it. Then, deploy your project using the Firebase CLI with the firebase deploy command. And just like that, your custom headers are now configured with Firebase Hosting.