使用Chromebug它的萤火虫扩展开发，女巫会给你充分的能力， firefox它自我和所有firefox扩展，如果它是一个网页:)

1-从这里安装最新版本：http://getfirebug.com/releases/chromebug/

2- Winkey + R打开run然后键入 firefox.exe -chromebug 这将运行firefox与chromebug

3-欲了解更多信息，请访问http://getfirebug.com/wiki/index.php/Chromebug

I'm trying to use Firebug to debug a JavaScript file in an extension I've added to Fx 4.
Can anyone tell me how to do it? I can only see website scripts.

Use Chromebug it's firebug for extension development, witch will give you the full ability to inspect debug firefox it self and all firefox extension as if it's a webpage :)

1- install the latest version from here: http://getfirebug.com/releases/chromebug/

2- Winkey+R to open run then type firefox.exe -chromebug this will run firefox with chromebug

3- for more information visit http://getfirebug.com/wiki/index.php/Chromebug