Looks like there is a related defect, but it's supposed to be fixed in 3.5. How are you triggering the vertical scroll, it seems from that defect that you need a touchpad to even trigger the event described? If you're not specifically after mousewheel events you could try the onscroll event instead, this works in FF3.5:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"><html dir="ltr" lang="en">
<html>
<head>
<style type="text/css">
body {
width: 2000px;
}
</style>
</head>
<body onscroll="console.log('Scrolled!')">
<p style="height: 2000px;">test</p>
</body>
</html>