This happens because in the earlier days of page design, dropdownlists were expected to be unaffected by layered DIV's (and modals?) because there was the chance that the values in the dropdownlist might be hidden and not be selectable.
If you are forced to code for older browsers, you might consider (especially if using ASP.NET AJAX) setting the DDL's Visible property to "False" to hide it when invoking the modal popup, and when the modal is hidden, set the DDL's Visible property to "True".
That's one example of an old-school workaround.
Hope this helps!
wil
Please click 'Mark as Answer' if my reply has assisted you.
"A common mistake that people make when trying to design something completely foolproof was to underestimate the ingenuity of complete fools." ~ Douglas Adams
http://wildobson.com