https://developer.mozilla.org/en/CSS/CSS_transitions
в Firefox 9.0.1 работает
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>…</title>
<style>
.trans {
opacity: 0;
-moz-transition: opacity 1s linear;
-webkit-transition: opacity 1s linear;
-o-transition: opacity 1s linear;
transition: opacity 1s linear;
}
</style>
</head>
<body>
<div id="foo">Foo</div>
<script src="http://code.jquery.com/jquery.js"></script>
<script>
$("#foo").bind("transitionend", function () {
alert("O_o");
}).addClass("trans");
</script>
</body>
</html>