Examine source code of Web Page Font Detector

{
  "manifest_version": 2,
  "name": "Web Page Font Detector",
  "version": "1.0",
  "description": "Add-on to detect fonts on web pages",
  "icons": {
    "48": "icons/48.png",
    "96": "icons/96.png",
    "128": "icons/128.png"
  },
  "permissions": [
    "tabs"
  ],
  "browser_action": {
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{19d9396c-3fb3-4762-a5c6-5b0a692222d4}"
    }
  }
}