Examine source code of Advanced Font Settings

{
  "name": "Advanced Font Settings",
  "version": "2.67",
  "manifest_version": 2,
  "description": "Customize per-script font settings.",
  "options_page": "options.html",
  "icons": {
    "16": "fonts16.png",
    "128": "fonts128.png"
  },
  "permissions": [
    "fontSettings"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{ee5268c9-a0ee-439b-920a-1684df6b9ac3}"
    }
  }
}