https://developer.mozilla.org/en-US/docs/Tools
Firefox Developer Tools is a set of web developer tools built into Firefox. You can use them to examine, edit, and debug HTML, CSS, and JavaScript.
This section contains detailed guides to all of the tools as well as information on how to debug Firefox for Android, how to extend DevTools, and how to debug the browser as a whole.
If you have any feedback on DevTools or want to contribute to the project, you can join the DevTools community.
Note: If you are just getting started with web development and using developer tools, our learning docs will help you — see Getting started with the Web and What are browser developer tools? for good starting points.
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. ( 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