Whether you do development using Extreme Programming and a test-first approach or whether you develop with the old fashioned waterfall method, there will be moments when your web application does not react in the way you expect or does not look like the designers said it should. You'll need tools and ingenuity to figure out what is wrong.
Probably still the best toolset for web developers to help them with the task of debugging their application is a combination of the Firefox browser with the Firebug and Web Developer plugins. They allow you to inspect and change almost anything on your page when you are testing your application. Within Firebug, you can see exactly what was sent and received between browser and server, set breakpoints, and so on. We have shown you some examples of Firebug usage in the earlier chapters.
Here is a list of the frequent problems that we encountered ourselves or that we saw in the forum on the Backbase developer network (http://bdn.backbase.com):
Debugging...