Examine source code of Pixel Ruler

{
  "manifest_version": 2,
  "name": "Pixel Ruler",
  "version": "1.0",
  "description": "A ruler for measure something in a website",
  "icons": {
    "48": "icons/48.png",
    "96": "icons/96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "pixel-ruler.js"
      ]
    }
  ],
  "permissions": [
    "activeTab"
  ],
  "applications": {
    "gecko": {
      "id": "anilsenay3@gmail.com-extension2"
    }
  }
}