Debugging with Visual Studio 2008

Enable script debugging in Tools->Internet Options.->Advanced->Disable Script Debugging (Other)

Visual Studio 2008 allows you to set client-side JavaScript breakpoints directly within your server-side .aspx files

Scripts dynamically generated on the server like the JSON-RPC proxies will also be available in the Script Document feature of the Visual Studio 2008 solution explorer view

Debugging with Mozilla Firefox

Download the Venkman JavaScript Debugger extension for Firefox here