You can use it to debug code running locally in Firefox or running remotely, for example on an Android device running Firefox for Android. See remote debugging to learn how to connect the debugger to a remote target.
The debugger ships inside Firefox, and these pages describe how to use the version that's embedded in Firefox. However, you can also run it as a standalone web application, and can then use it to debug code running in other browsers and in Node. For more details on that, see the project's GitHub repository.
The new debugger doesn't — yet — support all the features of the old debugger. See Limitations of the new debugger.
User Interface Tour
To find your way around the debugger, here's a quick tour of the UI.
To find out what you can do with the debugger, see the following how-to guides: