FireBug is like a supercharged version of Javascript Console. It helps with "debugging Javascript, DHTML and AJAX" and has been billed as a "combination of the Javascript Console, DOM Inspector, and a command line Javascript interpreter."

Among it’s features are XmlHttpRequest spy which allow you to see how websites are using XmlHttpRequest. I currently use LiveHTTPHeaders which supports standard requests as well as XmlHttpRequests. LiveHTTPHeaders is a really useful extension for debugging HTTP – for example it can help you work out where your cookie settings are wrong.

I also love the fact you can finally filter errors and hide CSS errors and internal errors and it’ll only show errors for the current page. Great stuff!