A useful Firefox extension to design your website is Firebug.
The extension gives you several development tools while you browse your Joomla website or other sites. Although you can edit, debug, and monitor CSS, HTML, and JavaScript in any web page, you cannot save the changes directly to your site. This you will have to do in the back end of Joomla or in an editor such as JEdit before you upload the changes with an FTP program (such as FireFTP, another Firefox extension).