Examine source code of Pixel Distance Ruler

{
  "manifest_version": 2,
  "name": "Pixel Distance Ruler",
  "version": "1.0",
  "description": "Click two points on the page to measure the pixel distance between them.",
  "icons": {
    "48": "icon.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_title": "Pixel Distance Ruler"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "style.css"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{2d755099-7bed-4958-bd3f-62bafdfa6819}"
    }
  }
}