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.
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.