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.
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, refer to the following how-to guides.
You probably want to pause the execution of your code, in order to see what is going on at various points. There are multiple ways to tell the Debugger how and when to pause:
What can you do after execution pauses?
You probably want to see the value of variables or expressions, either during execution or when it is paused.