Examine source code of DuckDuckGoogle

{
  "manifest_version": 2,
  "name": "DuckDuckGoogle (discontinued)",
  "version": "1.9",
  "author": "Jeroen Swen",
  "description": "Adds a 'Google it' button to the DuckDuckGo search engine. NOTICE: This add-on has been discontinued as of November 14, 2024. Please see the add-on page for more information.",
  "icons": {
    "96": "icons/icon-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://duckduckgo.com/?*",
        "https://safe.duckduckgo.com/?*"
      ],
      "js": [
        "js/main.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{5fe16324-5314-467a-8a68-568933cef8c0}"
    }
  }
}