This is a bug with the way the browser renders the page afaik. Forms appear ahead of DIV layers even though they shouldnt. Happens with me too.
http://www.macromedia.com/support/firew ... ve_con.htm
Kind of related...
Only solution AFAIK is to get all the forms to disappear when the popup menus appear. By no means impossible, just some added javascript for every page. It looks odd to me though. An example of this in action:
http://www.magicmushroom.org.uk/forum/active.asp [Hint: try operating the menus [:D]]