Examine source code of Word Count Tool

{
  "name": "Word Count Tool",
  "description": "Word Count Tool counts the number of words and characters in the selected text",
  "version": "4.1resigned1",
  "permissions": [
    "contextMenus",
    "notifications",
    "tabs",
    "storage",
    "http://*/*",
    "https://*/*"
  ],
  "background": {
    "persistent": false,
    "scripts": [
      "wordstats.js"
    ]
  },
  "manifest_version": 2,
  "icons": {
    "16": "icon16.png",
    "48": "icon48.png",
    "128": "icon.png"
  },
  "homepage_url": "https://wordcounttools.com",
  "offline_enabled": true,
  "browser_specific_settings": {
    "gecko": {
      "id": "jid0-YHLk2psjhEWXNJqMKTU7dDcMJcN@jetpack"
    }
  }
}