A friend recently pointed me to a useful development toolbar for Firefox. It’s similar to the development toolbar here but does even more stuff.
You can find the toolbar at http://chrispederick.com/work/webdeveloper/.
Note: See the May 2008 update at the bottom of this article.
alert("You are here") around the code to find out what’s going on. The problems with this approach are that you can’t easily switch them on and off without going through and commenting out individual calls; the fact that you get a dialog box up which needs to be accepted by the user can affect time-sensitive code, so you might not be able to debug it using this method; and finally it’s difficult to compare values from one
alert to the next since the text just comes up in a dialog box.