Examine source code of Log active element

{
  "manifest_version": 2,
  "name": "ActiveEl",
  "version": "1.1",
  "description": "Add a console.log when active element changes.",
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "index.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{15c1a0bc-aec9-404c-8567-91afc4a5588a}"
    }
  }
}