Examine source code of Display Window Size

{
  "manifest_version": 2,
  "name": "Display Window Size",
  "version": "1.0",
  "description": "Display window size for the website you are accessing.",
  "permissions": [
    "activeTab"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "display-window-size-extension@yourdomain.com"
    }
  }
}