You can open the Firefox Developer Tools from the menu by selecting Tools > Web Developer > Web Developer 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. Visualize 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.
Provides a means to access the page's accessibility tree, allowing you to check what's missing or otherwise needs attention.
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.
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.