For Web Developers

Mozilla is by far the best platform for web developers to build standards-compliant sites with. The tools which have been built on top of the Mozilla framework make developing and debugging websites, particularly complex or dynamic ones, an order of magnitude easier than in other environments.

Venkman - JavaScript Debugger Venkman is a JavaScript Debugging environment for Mozilla-based browsers. It allows you to debug JavaScript code running in a web page in exactly the same way that you might debug C++ code running under your OS. It features breakpoints, call stack inspection and variable/object inspection, and allows execution of arbitrary JavaScript code.
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DOM Inspector The DOM Inspector is a unique tool that can be used to inspect and edit the live DOM of any web document, allowing you to see the page exactly as the browser sees it. The DOM hierarchy can be navigated using a two-paned window that allows for a variety of different views on the document and all nodes within.
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JavaScript Console Our JavaScript Console gives exact details of any errors found in scripts in the page, along with a hyperlink to the source file. You can also execute arbitrary JavaScript via the type-in, see strict JavaScript warnings, and <xsl:message> outputs in your XSLT.

The above three tools are available from the Tools | Web Development menu if you opted for a Full installation of Mozilla.

Web standards reference sidebars Sidebars containing references to the standards for almost every web technology you can think of, from CSS to XSLT, including HTML 4.01, XHTML, and DOM 2. These small sidebars keep the specs at your fingertips, enabling you to look up the syntax for or status of any element in a second.
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Mozilla Source Generator Have you ever wanted to see the source of your page after scripts have executed? Check out the Mozilla Source Generator, which generates formatted HTML source from the current DOM of the page.
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There is a long list of documentation for web developers working with Mozilla and W3C technologies.