The following works in IE, but not Firefox:
var el = $get('divToMask');
var box = Sys.UI.DomElement.getBounds(el);
var maskEl = $get('maskDiv');
// Only seems to work in IE
maskEl.style.width = box.width;
maskEl.style.height = box.height;
Sys.UI.DomElement.setLocation(maskEl, box.x, box.y);
box.width and box.height contain the correct values, but Firefox ignores the maskEl.style .width/.height methods.