Examine source code of Day Counter

{
  "manifest_version": 2,
  "name": "Day Counter",
  "version": "1.0",
  "description": "Displays elapsed and remaining days of a year.",
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{d6b8a3ce-d298-41a0-8c5a-c43f405f7dc7}"
    }
  }
}