XUL <expander> had been used for the Preference window on Firefox 1.0. Since Firefox 1.5, it have not been used.

Firefox 1.0's Preference Window (Prefwindow IV)

<expander> element still exist at chrome://global/content/xul.css in Firefox 3.7a5. We seem to continue to use it. 

expander {
  -moz-binding: url("chrome://global/content/bindings/expander.xml#expander");
  -moz-box-orient: vertical;
}

But today no <expander> is used on mozilla-central (firefox main stream). I searched for "<expander" on MXR [*], and no result found.

In addition, <expander> element is not listed at "XUL Reference" page at MDC.

So I'm scheduled to remove <expander> from Configuration Mania in the next release.