FF mouseleave/mouseenter events

Question

I write extension to FF and this part of the code does not work on earlier versions 6.0-8.0. But on 12.0-13.0 versions everything works great. What is the reason?

        for(var i = 0; i < doc.getElementsByClassName('repInfo').length; i++)
        {
            doc.getElementsByClassName('repInfo')[i].getElementsByTagName('a')[0].style.visibility = "hidden";
            doc.getElementsByClassName('repInfo')[i].getElementsByTagName('a')[1].style.visibility = "hidden";

            doc.getElementsByClassName('repInfo')[i].addEventListener('mouseleave', 
                function(e) {
                    e.target.getElementsByTagName('a')[0].style.visibility = "hidden";
                    e.target.getElementsByTagName('a')[1].style.visibility = "hidden";
                }, false);
            doc.getElementsByClassName('repInfo')[i].addEventListener('mouseenter', 
                function(e) { 
                    e.target.getElementsByTagName('a')[0].style.visibility = "visible";
                    e.target.getElementsByTagName('a')[1].style.visibility = "visible";
                }, false);
        }

Answer 1

Firefox 10添加了mouseenter和mouseleave支持。