Examine source code of Header Modifier

{
  "manifest_version": 3,
  "name": "Header Modifier",
  "version": "1.1.0",
  "description": "Inject custom headers for specific URLs",
  "permissions": [
    "declarativeNetRequest",
    "declarativeNetRequestWithHostAccess",
    "storage"
  ],
  "host_permissions": [
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{22fe3f4e-3cbe-4f10-a726-84ac6dbe31c5}",
      "strict_min_version": "142.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ],
        "optional": []
      }
    }
  },
  "action": {
    "default_popup": "popup.html",
    "default_title": "Header Modifier",
    "default_icon": {
      "128": "images/128.png"
    }
  },
  "icons": {
    "128": "images/128.png"
  },
  "declarative_net_request": {
    "rule_resources": [
      {
        "id": "ruleset_1",
        "enabled": true,
        "path": "rules.json"
      }
    ]
  }
}