Examine source code of Font Pairing Preview

{
  "manifest_version": 2,
  "name": "Font Pairing Preview",
  "version": "1.0",
  "description": "Apply and preview custom font pairings on live web pages.",
  "icons": {
    "48": "icon.png"
  },
  "permissions": [
    "activeTab",
    "storage"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{21494238-8367-4119-a7e1-e235908eceac}"
    }
  }
}