If you develop for the web, there is a new extension for Firefox that you simply must get:

Firebug combines the functionality of the DOM inspector with the Error console and command-line JavaScript interpretor and then packs it with some great additional features.

Among its many features, Firebug allows you to inspect XMLHttpRequest responses, and so it will be invauable when debugging Ajax applications.

Another great feature is the ability to log custom debug messages from your javascript. Until now, debugging JavaScript often involved strategically placed alert messages. With Firebug, you simply add the following code to your script:

function printfire()
{
    if (document.createEvent)
    {
        printfire.args = arguments;
        var ev = document.createEvent("Events");
        ev.initEvent("printfire", false, true);
        dispatchEvent(ev);
    }
}

Then you call printfire(something); in your script and your debug message will be logged in the Firebug console.

You can read about Firebug’s other features in the FAQ .

Congratulations to programmer Joe Hewitt for this great plugin!