Follow the instructions here for enabling the browser (as opposed to content) debugger: https://developer.mozilla.org/en-US/docs/Debugging_JavaScript#JavaScript_Debugger
Once your scripts are loaded in the browser, they should become visible in the debugger's sources pane (but not before that).
If you still can't get that to work, I would love a test case and detailed steps to reproduce, so I could try and diagnose the problem :)