Examine source code of Catch The Links

{
  "manifest_version": 2,
  "name": "Catch The Links",
  "version": "1.0",
  "description": "Links fall like rain. Use the mouse 'catcher' to catch as many falling links as you can.",
  "permissions": [
    "storage",
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_action": {
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{2db2de48-74df-4393-b82f-535dda1f8f75}"
    }
  }
}