Examine source code of Inspector

{
  "manifest_version": 2,
  "name": "Inspector",
  "version": "1.1.2",
  "description": "Inspect HTML elements, CSS box model properties and their details",
  "permissions": [
    "activeTab",
    "tabs",
    "scripting"
  ],
  "icons": {
    "16": "icons/16.png",
    "48": "icons/48.png",
    "128": "icons/128.png"
  },
  "background": {
    "scripts": [
      "polyfill.js",
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_icon": {
      "16": "icons/16.png",
      "48": "icons/48.png",
      "128": "icons/128.png"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{abccda5b-8b15-4cc9-908d-65cf142767e3}"
    }
  }
}