Examine source code of Display Tab Index

{
  "manifest_version": 2,
  "name": "Display Tab Index",
  "version": "1.0",
  "description": "Displays the tabindex attribute values of elements on the current webpage.",
  "permissions": [
    "tabs"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "displaytabindex@gmail.com"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  }
}