Examine source code of Live Font Inspector

{
  "manifest_version": 2,
  "name": "Live Font Inspector",
  "description": "Show font-family, size, weight and color on hover",
  "version": "1.0",
  "icons": {
    "32": "32.png",
    "64": "64.png",
    "128": "128.png"
  },
  "permissions": [
    "<all_urls>"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{e3e6a2f0-22d5-4822-abae-22fbf40713ed}",
      "strict_min_version": "90.0"
    }
  }
}