Note: This page describes devtools APIs as they exist in Firefox 55. Although the APIs are based on the Chrome devtools APIs, there are still many features that are not yet implemented in Firefox, and therefore are not documented here. To see which features are currently missing please see Limitations of the devtools APIs.
You can use WebExtensions APIs to extend the browser's built-in developer tools. To create a devtools extension, include the "devtools_page" key in manifest.json:
"devtools_page": "devtools/devtools-page.html"
The value of this key is a URL pointing to an HTML file that's been bundled with your extension. The URL should be relative to the manifest.json file itself.
The HTML file defines a special page in the extension, called the devtools page.