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.