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View and edit page content and layout. Visualise many aspects of the page including the box model, animations, and grid layouts.
See the network requests made when a page is loaded.
Responsive Design Mode
See how your website or app will look and behave on different devices and network types.
These developer tools are also built into Firefox. Unlike the "Core Tools" above, you might not use them every day.
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.
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.
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.
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.
If you want to help to improve the developer tools, these resources will get you started.