Examine source code of Pixel Ruler on Image

{
  "manifest_version": 2,
  "name": "Pixel Ruler on Image",
  "version": "1.0",
  "description": "Hiển thị thước pixel khi rê chuột lên ảnh.",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "ruler.js"
      ]
    }
  ],
  "icons": {
    "48": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{98d82b6a-14d7-44fc-bd1d-0a71438e55ca}"
    }
  }
}