Examine source code of Font Size Peek

{
  "manifest_version": 2,
  "name": "Font Size Peek",
  "version": "1.0",
  "description": "Show a small tooltip with font-family and font-size when hovering over text.",
  "icons": {
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ],
      "run_at": "document_idle"
    }
  ],
  "permissions": [],
  "browser_specific_settings": {
    "gecko": {
      "id": "{d33742ba-af36-4d0e-a79b-fc498f102504}"
    }
  }
}