Firefox 3b1 queryCommandEnabled( "formatblock") throws NS_ERROR_NOT_IMPLEMENTED... works in Firefox 2

Running the following example in Firefox 3b1 throws a
NS_ERROR_NOT_IMPLEMENTED exception.
The same code returns "true" in Firefox 2.

<script>
	var error;
	document.designMode='on';
	try {
		alert(	document.queryCommandEnabled( "formatblock" ) );
	}
	catch( error ) {
		alert( error );
	}
</script>


Does anyone have any insight into what might have changed in this
regard?