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.

See what your users see in their browsers. Test your sites in up to 800 browser and OS combinations with our cross-browser testing tutorial.

Made just for you by Mozilla and Sauce labs.

Test on Sauce Labs

The Core Tools

Page Inspector

View and edit page content and layout. Visualise many aspects of the page including the box model, animations, and grid layouts.

Web Console

See messages logged by a web page and interact with the page using JavaScript.

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.

Performance Tools

Analyze your site's general responsiveness, JavaScript, and layout performance.

Responsive Design Mode

See how your website or app will look and behave on different devices and network types.

More Tools

These developer tools are also built into Firefox. Unlike the "Core Tools" above, you might not use them every day.

Memory
Figure out which objects are keeping memory in use.
Storage Inspector
Inspect cookies, local storage, indexedDB, and session storage present in a page.
DOM Property Viewer
Inspect the page's DOM properties, functions, etc.
Developer Toolbar
A command-line interface for the developer tools.
Eyedropper
Select a color from the page.
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.
Taking screenshots
Take a screenshot of the entire page or of a single element.

Connecting the Developer Tools

If you open the developer tools using keyboard shortcuts or the equivalent menu items, they'll target the document hosted by the currently active tab. But you can attach the tools to a variety of other targets, too, both within the current browser and in different browsers or even different devices.

about:debugging
Debug add-ons, content tabs, and workers running in the browser.
Connecting to Firefox for Android
Connect the developer tools to an instance of Firefox running on an Android device.
Connecting to iframes
Connect the developer tools to a specific iframe in the current page.
Connecting to other browsers
Connect the developer tools to Chrome on Android and Safari on iOS.

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.

Example devtools add-ons
Use these examples to understand how to implement a devtools add-on.
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.

Contribute

If you want to help to improve the developer tools, these resources will get you started.

Get Involved
Mozilla wiki page explaining how to get involved.
firefox-dev.tools
A tool helping to find bugs to work on.

Join the Developer tools community

Choose your preferred method for joining the discussion: