Examine source code of Dual URL Viewer

{
  "manifest_version": 2,
  "name": "Dual URL Viewer",
  "description": "Open two different URLs or parts of the same URL side by side",
  "version": "3.0",
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icon.png"
    },
    "default_title": "Dual URL Viewer"
  },
  "permissions": [
    "tabs",
    "storage",
    "<all_urls>"
  ],
  "icons": {
    "16": "icon.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "dual-url-viewer@example.com",
      "strict_min_version": "58.0"
    }
  }
}