Examine source code of DEV-Tools

{
  "description": "Adds a new sidebar to the developer tools inspector panel. The sidebar displays the page document as an inspectable object.",
  "manifest_version": 2,
  "name": "DEV-Tools",
  "version": "8.9.0",
  "permissions": [
    "<all_urls>"
  ],
  "devtools_page": "devtools/devtools-page.html",
  "browser_specific_settings": {
    "gecko": {
      "id": "{c5ecb7b9-6519-4e8b-8a76-e71e0b4b5624}"
    }
  }
}