If you haven't yet, go get the Firebug 0.4 extension for Firefox from Joe Hewitt. Roger Johansson mentioned it, and Justin Palmer posted an in-depth walk through of the many really great features in Firebug.

The days of using alert() for debugging are over (except, perhaps, in other browsers. You can log messages and objects to the console (at info, warn, debug and error levels), trace execution, and even fire up a breakpoint with the keyword 'debugger'! Not to mention all the typical error reporting stuff and DOM walkthrough features you'd expect. Go check it out!