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Firefox Developer Tools

Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile. For the latest updates to the developer tools, download Firefox Developer Edition.


Authoring tools for websites and web apps.

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.

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.
Select a color from the page.
A dashboard to debug add-ons and workers
Working with iframes
How to target a particular iframe.


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App Manager has been replaced by WebIDE.
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.
Connect the developer tools to Chrome on Android and Safari on iOS


Diagnose and fix performance problems.

Performance Tool
Analyze your site's general responsiveness, JavaScript, and layout performance.
Figure out which objects are keeping memory in use.
Frame rate graph
See the frame rate for your site.
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.


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.
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

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.
JavaScript linting and code analysis tool.

Join the Developer tools community

Choose your preferred method for joining the discussion: