FireBug

FireBug is a really good add-on for Firefox that allows you to depurate the code of your webapps without having to close Firefox. Thanks to Firebug, you will be able to edit HTML code and check how it works instantaneously, without having to reload. That same action can be performed on stylesheets (CSS) and you'll also check changes on the fly. Firebug includes a lite ruler that will be reallyuseful when working on the design of the application. Other interesting features of firebug are: - Network activity monitoring. - Javascript code checker. - DOM explorer (Document Object Model). - etc.