Firefox Iframes and pdfs

I have built an asp.net page using a master page as a header. The master page contains the asp:menu control.

The main page shows a iframe. The iframe is added dynamically to a placeholder and shows pdf files. The problem that I have is in Firefox the drop down menu options are hidden behind the iframe; in IE the menu options show properly.

As you can see I did add a z-index to the code but it made no difference.

strHTML = "";
//add iframe to previously defined placeholder
plhIFrame.Controls.Add(new LiteralControl(strHTML));

Has anyone any idea how to overcome this problem?