The very handy Firebug javascript debugger for Firefox has just released a major update. Click here to get it.
If you aren’t using Firebug, you’re missing out. It is hands-down the best javascript debugger out there.
It can intercept and log all XMLHttpRequests, you can set breakpoints, inspect and modify the DOM, execute javascript on the fly with the console, and a lot more. New features allow you to tweak and visualize CSS metrics (with rulers), and monitor network activity.
Seriously, what are you waiting for? Go get it.