Examine source code of DateTimeDisplay

{
  "manifest_version": 2,
  "name": "DateTimeDisplay",
  "version": "1.0",
  "description": "Displays the current date and time in the toolbar.",
  "icons": {
    "48": "icon.png"
  },
  "browser_action": {
    "default_icon": {
      "48": "icon.png"
    },
    "default_popup": "popup.html"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{ea70fd7a-4443-4f73-b771-8dba4f950935}"
    }
  }
}