Joe Hewitt is working on updating his excellent Firebug Firefox extension. If you’re not familiar with it, you should be. It provides detailed per-page error tracking, XMLHTTPRequest tracking, DOM Navigation – everything that a Javascript developer needs.
The new version of Firebug is adding one more feature: Javascript debugging. This will allow you to set break points in your code, pause the application, restart it, and all sorts of cool stuff. If this sounds interesting to you, you should contact Joe, as he needs testers to help him get this off the ground.