Examine source code of Word Counter2

{
  "manifest_version": 2,
  "name": "Word Counter",
  "version": "1.0",
  "description": "This add-on counts words and characters on the current webpage or selected text.",
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{92760649-ed03-4831-b5ed-26312db5f472}"
    }
  }
}