Examine source code of Page Ruler Tool

{
  "manifest_version": 2,
  "name": "Page Ruler Tool",
  "version": "1.2",
  "description": "Draw a ruler to measure elements on any webpage.",
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/logo_64.png",
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "style.css"
      ],
      "run_at": "document_idle"
    }
  ],
  "icons": {
    "32": "icons/logo_32.png",
    "64": "icons/logo_64.png",
    "128": "icons/logo_128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{71bc2f80-6c6b-4a36-8778-1d036854a650}"
    }
  }
}