Firefox Developer Tools

Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile

What's new in Firefox Developer Edition?

Firefox Developer Edition is a version of Firefox tailored for developers, featuring the latest Firefox features and experimental developer tools. The current Developer Edition includes these updates to the developer tools:

Creating

Authoring tools for websites and web apps.

Scratchpad: A text editor built into Firefox that lets you write and execute JavaScript.
Style Editor: View and edit CSS styles for the current page.
Shader Editor: View and edit the vertex and fragment shaders used by WebGL.
Web Audio Editor: Examine the graph of audio nodes in an audio context, and modify their parameters.

Exploring and debugging

Examine, explore, and debug websites and web apps.

Console: See messages logged by a web page and interact with the page using JavaScript.
Page Inspector: View and modify a page in HTML and CSS.
JavaScript Debugger: Stop, step through, examine, and modify the JavaScript running in a page.
Network Monitor: See the network requests made when a page is loaded.
Storage Inspector: Inspect cookies, local storage, indexedDB, and session storage present in a page.
DOM Inspector: Inspect the page's DOM properties, functions, etc.
Developer Toolbar: A command-line interface for the developer tools.
Remote Debugging: Connect the developer tools to Firefox from remote machine.
Eyedropper: Select a color from the page.
about:debugging: A dashboard to debug add-ons and workers
Working with iframes: How to target a particular iframe.

Mobile

Tools for mobile development.

Responsive Design Mode: See how your website or app will look on different screen sizes without changing the size of your browser window.
App Manager: App Manager has been replaced by WebIDE.
WebIDE: Create, edit, run, and debug web apps using the Firefox OS Simulator or a real Firefox OS device. WebIDE is the replacement for the App Manager, available from Firefox 33 onwards.
Firefox OS Simulator: Run and debug your Firefox OS app on the desktop, without needing a real Firefox OS device.
Debugging on Firefox for Android: Connect the developer tools to Firefox for Android.
Debugging Firefox for Android with WebIDE: For Desktop Firefox 36+ / Android Firefox 35+, there's a simpler process.
Valence: Connect the developer tools to Chrome on Android and Safari on iOS

Performance

Diagnose and fix performance problems.

Performance Tool: Analyze your site's general responsiveness, JavaScript, and layout performance.
Memory: Figure out which objects are keeping memory in use.
Frame rate graph: See the frame rate for your site.
Waterfall: Figure out what the browser is doing as it runs your site.
Call Tree: Figure out where your JavaScript code is spending its time.
Flame Chart: See which functions are on the stack over the course of a performance profile.
Paint Flashing Tool: Highlights the parts of the page that are repainted in response to events.
Reflow Event Logging: See reflow events in the web console.
Network Performance: See how long the parts of your site take to load.

Debugging the browser

By default, the developer tools are attached to a web page or web app. But you can also connect them to the browser as a whole. This is useful for browser and add-on development.

Browser Console: See messages logged by the browser itself and by add-ons, and run JavaScript code in the browser's scope.
Browser Toolbox: Attach the Developer Tools to the browser itself.

Extending the devtools

The developer tools are designed to be extensible. Firefox add-ons can access the developer tools and the components they use to extend existing tools and add new tools. With the remote debugging protocol, you can implement your own debugging clients and servers, enabling you to debug websites using your own tools or to debug different targets using the Firefox tools.

Add a new panel to the devtools: Write an add-on that adds a new panel to the Toolbox.
Remote Debugging Protocol: The protocol used to connect the Firefox Developer Tools to a debugging target like an instance of Firefox or a Firefox OS device.
Source Editor: A code editor built into Firefox that can be embedded in your add-on.
The Debugger Interface: An API that lets JavaScript code observe the execution of other JavaScript code. The Firefox Developer Tools use this API to implement the JavaScript debugger.
Web Console custom output: How to extend and customize the output of the Web Console and the Browser Console.
Example devtools add-ons: Use these examples to understand how to implement a devtools add-on.

Contribute

If you want to help to improve the developer tools, there are several resources were to start.

Get Involved: Mozilla wiki page explaining how to get involved.

firefox-dev.tools: A tool helping to find bugs to work on.

More resources

This section lists resources which are not currently maintained by Mozilla's developer tools team, but which are widely used by web developers. We've included a few Firefox add-ons in this list, but for the complete list see the “Web Development” category on addons.mozilla.org.

Firebug: A very popular and powerful web development tool, including a JavaScript debugger, HTML and CSS viewer and editor, and network monitor.
DOM Inspector: Inspect, browse, and edit the DOM of web pages or XUL windows.
Web Developer: Adds a menu and a toolbar to the browser with various web developer tools.

Webmaker Tools: A set of tools developed by Mozilla, aimed at people getting started with Web development.
W3C Validators: The W3C website hosts a number of tools to check the validity of your website, including its HTML and CSS.
ESLint: JavaScript linting and code analysis tool.

Join the Developer tools community

Choose your preferred method for joining the discussion:

IRC: #devtools (learn more)
Team info: Dev tools wiki
Blog: Hacks blog