The story is at Mozillaquest.com. "Composer is the free, open source, HTML editor and Web authoring module of the Netscape 6 and Mozilla browser-suites. It helps you to create and to edit Web page, e-mail, and text-content-oriented documents easily.
The Composer desktop looks and feels much as a graphical word-processor desktop looks and feels. However, behind the scenes, Composer is formatting your Web page document in HTML for publication on the World Wide Web, for e-mail, or as an attribute-rich, text-content-oriented document -- or as plain text if you like.
Composer is an easy-to-use, WYSIWYG (What You See Is What You Get), graphical user interface (GUI), HTML editor. You also can view, write and edit HTML source code with Composer."