Examine source code of CSS Live Tester

{
  "manifest_version": 2,
  "name": "CSS Live Tester",
  "version": "1.0",
  "description": "Live test custom CSS on the current page.",
  "permissions": [
    "activeTab",
    "tabs"
  ],
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "icons": {
    "48": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{b74d9d7d-7e3d-4bdb-9148-bd9e1ea7e9c5}"
    }
  }
}