Firefox drop down list disables on postback with update panel

Question

I have a problem in FireFox (3, 3.5...) browser with using AJAX Update Panel. There are DropDownList with some values and Button in the UpdatePanel. Update Panel works good in FireFox. So, after you click on Button, DropDownList stick, it’s impossible to change value in DropDownList.

if change focus to another window and then back to FF then it works fine.

I fixed it with:

ScriptManager.RegisterStartupScript(this.Button2, Button2.GetType(), "CodeAlert", "window.setTimeout(\"alert('Your code is expired at "+DateTime.Now.ToString()+"')\",0);",  true);

But it's possible fix that without an alert message?