Examine source code of Word Count is

{
  "manifest_version": 2,
  "name": "Word Count",
  "version": "1.0",
  "description": "Count words on the current page with one click.",
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{a985b9d7-7b0c-4d35-9c05-e6a94da78fc7}"
    }
  }
}