Examine source code of TheWordCounter

{
  "manifest_version": 2,
  "name": "WordCounter",
  "version": "1.0.0",
  "description": "Count words, characters, paragraphs and lines in any text",
  "author": "XMP <lanhanh64529@gmail.com>",
  "browser_specific_settings": {
    "gecko": {
      "id": "wordcounter@lanhanh64529.gmail.com"
    }
  },
  "icons": {
    "16": "icons/icon16.png",
    "32": "icons/icon32.png",
    "48": "icons/icon48.png",
    "128": "icons/icon128.png"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_title": "WordCounter",
    "default_icon": {
      "16": "icons/icon16.png",
      "32": "icons/icon32.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "permissions": []
}