If you are familiar with Firebug, then this is probably all old news. In my opinion Firebug is hands down the best HTML and CSS web debugging tool you can have in your toolkit.
A quick summary
Some client side action on the web page takes place and requires some sort of server side action. For example retrieving and filling a drop down list with values based on a previous selection, a common way of selecting a category from a tree structure that is to large to render in a single drop down.
After fixing the Web Service I place a break point in the script section of Firebug immediately after the Web Service returns. I select the result variable and add it to the watch, after visually inspecting the JSON in the Watch panel, I can see the data structure is slightly different from what I was expected.
In the response section of Firebug you can see the data passed back from the Web Service.
You can see with Firebug it is very easy to see exactly what is wrong.
The benefits of using a Web Service