it's because Firefox considers the whitespace between element nodes to be text nodes (whereas IE does not) and therefore using .nextSibling on an element gets that text node in Firefox.

It's useful to have a function to use to get the next element node. Something like this

/* 

   Credit to John Resig for this function 

   taken from Pro JavaScript techniques 

*/

function next(elem) {

    do {

        elem = elem.nextSibling;

    } while (elem && elem.nodeType !== 1);

    return elem;        

}

then you can do

var elem = document.getElementById('the_id');

var nextElem = next(elem); 



if (nextElem) 

    nextElem.style.display = 'none';