Obsolete since Gecko 7.0 (Firefox 7.0 / Thunderbird 7.0 / SeaMonkey 2.4)
This feature is obsolete. Although it is still supported by browsers, its usage is discouraged in new projects. Try to avoid using it.
Prior to Gecko 7.0 this method showed up as present, due to bug bug 9850, it always only returned null. The only workaround is to create a text node, CDATA section, attribute node value, etc. which has the value referred to by the entity, using Unicode escape sequences or fromCharCode() as necessary.