Exploring the features of Firebug, with code examples covering using the console, timing code, stack traces, debugging and breakpoints, profiling, tracing HTTP requests. An excellent overview of Firebug's functionality
Webkit have released Web Inspector, which is a Firebug-type tool for Webkit and Safari. Contains Firebug related features like categorising resources, JavaScript console, HTTP network monitor, load time and resource size monitors, inline JavaScript and HTML error reporting.
All JavaScript implementations implement a debugger statement, which can be called at any point in your script. Also we have console.debug in Firebug.
Apart from being an elegant debugger, FireBug offers the developer hooks that can be used from JavaScript, offering logging, assertions, performance measuring, and command line functions for inspection or traversing a document.
All of the tools you need to poke, prod, and monitor your JavaScript, CSS, HTML and Ajax are brought together into one seamless experience, including a debugger, error console, command line, and a variety of fun inspectors.