Examine source code of Page Ruler

{
  "manifest_version": 2,
  "name": "Page Ruler",
  "version": "1.0.0",
  "description": "Measure elements on any webpage with pixel-perfect rulers and dimension tools",
  "permissions": [
    "activeTab",
    "<all_urls>"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon-16.png",
      "32": "icons/icon-32.png",
      "48": "icons/icon-48.png",
      "128": "icons/icon-128.png"
    }
  },
  "icons": {
    "16": "icons/icon-16.png",
    "32": "icons/icon-32.png",
    "48": "icons/icon-48.png",
    "128": "icons/icon-128.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "content.css"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "e15f656bc41cd533ba9db21bcd45b822c3dba06b@extension.com",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  }
}