Obsolete since Gecko 19 (Firefox 19 / Thunderbird 19 / SeaMonkey 2.16)
This feature is obsolete. Although it is still supported by browsers, its usage is discouraged in new projects. Try to avoid using it.
Note: Support for XForms was removed from Firefox as of Firefox 19. See the HTML Forms Guide for approaches to creating forms in HTML5.
An online book giving you a guided tour of XForms.
XForms in Wikibook Format - Over 50 examples tested with Firefox.
XForms support can be added to Firefox and SeaMonkey by installing the Mozilla XForms extension. This extension, while supporting a significant subset of the XForms 1.0 and 1.1 candidate recommendations, is not actively maintained any more since about 2010. The last official release has been done for Firefox 3.6 and is available for download on addons.mozilla.org.
For more details about the future of the Mozilla XForms extension see this blog post.