Ok, this is a not so orthodox CSS switch for Firefox to IE, its far from standards and will not pass W3C verification, but its very useful when embedding applications, such as OsCommerce (STS 4.5.8), into XHTML crossbrowser templates.

The underscore ( _ ) is ignored by any browser but IE so…
#menu {
position: fixed;
_position: absolute;
}
More info on this one