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 just getting started with 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
You can open the Firefox Developer Tools from the menu by selecting Tools > Web Developer > Toggle Tools or use the keyboard shortcut Ctrl + Shift + I or F12 on Windows and Linux, or Cmd + Opt + I on macOS.
The ellipsis menu on the right-hand side of Developer Tools, contains several commands that let you perform actions or change tool settings.
|This button only appears 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|
View and edit page content and layout. Visualise many aspects of the page including the box model, animations, and grid layouts.
Note: The collective term for the UI inside which the DevTools all live is the Toolbox.
These developer tools are also built into Firefox. Unlike the "Core Tools" above, you might not use them every day.
- 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.
- Select a color from the page.
- Style Editor
- View and edit CSS styles for the current page.
- 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.
- Overlay horizontal and vertical rulers on a web page
For the latest developer tools and features, try 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.
- 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.
Extending the devtools
Migrating from Firebug
Firebug has come 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.
If you want to help improve the developer tools, these resources will get you started.