You can force the menu to work by overwrite the Page_PreInit method on the page and telling the page that the client target is a modern “uplevel” browser:-
protected void Page_PreInit(object sender, EventArgs e)
Page.ClientTarget = "uplevel";
Please note that you have to do this in the page class file, and not in the master page class file. Obviously this means you have to do it for every page – which is not ideal.
To get round this limitation simply make a class which inherits from System.Web.UI.Page, and change all your pages to inherit from this class instead (a find and replace is ideal here!).