Obsolete since Gecko 19 (Firefox 19 / Thunderbird 19 / SeaMonkey 2.16) This feature is obsolete.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.
XForms were envisioned as the future of online forms as envisioned by the W3C. Drawing on other W3C standards like XML Schema, XPath, and XML Events, XForms tries to address some of the limitations with the current HTML forms model. Other strengths that XForms brings to the table is the separation of data from presentation, strong data typing, the ability to submit XML data to servers instead of name/value pairs, and a descriptive way to author forms so that they can be displayed by a wide variety of devices. XForms is a W3C specification.
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.