Firefox Developer Tools

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

If you are looking for information on using the web developer tools available in Firefox, you've come to the right place — this page provides links to detailed information on all of the core tools and additional tools, and further information such as how to connect to and debug Firefox for Android, how to extend the devtools, and how to debug the browser as a whole.

Please explore the links found in the sidebar, and further down the page. If you have any feedback or questions about the devtools, send us messages on our mailing list or IRC channel (see the community links near the bottom of the page). If you have any feedback or questions specifically about the documentation, the MDN discourse is a good place to post.

Note: If you are a beginner to web development and using developer tools, our learning web development docs will help you — see Getting started with the Web and What are browser developer tools? for good starting points.

The Core Tools

The right-hand side of the Developer Tools toolbar, which can be opened with Ctrl + Shift + I on Windows and Linux, or Cmd + Opt + I on OS X, contains several buttons that let you perform actions or change tool settings.

	This button will only appear when there are multiple iframes on a page. Click it to display a list of the iframes on the current page and select the one with which you want to work.
	Click this button to take a screenshot of the current page. (Note: This feature is not turned on by default and must be enabled in settings before the icon will appear.)
	Toggles Responsive Design Mode.
	Opens the menu that includes docking options, the ability to show or hide the split console, and Developer Tools settings. The menu also includes links to the documentation for Firefox Web Tools and the Mozilla Community.
	Closes the Developer 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.

Note: The collective term for the UI inside which the DevTools all live is the Toolbox.

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.
Accessibility inspector: Provides a means to access the page's accessibility tree, allowing you to check what's missing or otherwise needs attention.
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.
Measure a portion of the page: Measure a specific area of a web page.
Rulers: Overlay horizontal and vertical rulers on a web page

For the latest developer tools and features, try Firefox Developer Edition.

Download Firefox Developer Edition

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.

Migrating from Firebug

Firebug is coming to the end of its lifespan (see Firebug lives on in Firefox DevTools for details of why), and we appreciate that some people will find migrating to another less familiar set of DevTools to be challenging. To ease a transition from Firebug to the Firefox developer tools, we have written a handy guide — Migrating from Firebug.

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:

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