Examine source code of Page Ruler

{
  "manifest_version": 2,
  "name": "Page Ruler",
  "version": "3.0",
  "description": "Measure elements and distances on web pages",
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icon.png",
      "48": "icon.png",
      "128": "icon.png"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{63f0a4af-4930-4984-8ba5-c84b95f11050}"
    }
  }
}