For light javascript programming/testing, JSFiddle is a good online tool. It can be used with firebug also.
- Go to http://jsfiddle.com
- Add "https://getfirebug.com/firebug-lite-debug.js" to External Resources
Or, use this link -- FireBug is already added, http://jsfiddle.net/keithkim/m0e8515u/