Examine source code of Font Resizer

{
  "manifest_version": 2,
  "name": "Font Resizer",
  "version": "1.3",
  "description": "Resize font on any website to enhance readability.",
  "permissions": [
    "activeTab",
    "storage"
  ],
  "icons": {
    "32": "icons/logo_32.png",
    "64": "icons/logo_64.png",
    "128": "icons/logo_128.png"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icons/logo_64.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{94648fe2-1808-4457-add1-63640262a4f4}"
    }
  }
}