I have an aspx page with UltraWebGrid v 10.3 (10.3.20103.2164) which I am able to make scrollable in IE 8/9 and Firefox by the following code:

            gridMain.DisplayLayout.StationaryMargins = 

                _initHeaderFooter != null ? StationaryMargins.HeaderAndFooter :

                StationaryMargins.Header;

            gridMain.DisplayLayout.BorderCollapseDefault = BorderCollapse.NotSet;

            gridMain.DisplayLayout.ScrollBar = ScrollBar.Always;

            gridMain.DisplayLayout.ScrollBarView = ScrollBarView.Vertical;

            gridMain.DisplayLayout.FrameStyle.CustomRules = "table-layout:fixed";

            gridMain.DisplayLayout.FrameStyle.Height = new Unit(250);

            gridMain.DisplayLayout.FixedHeaderStyleDefault.CssClass = "invWebTableFixedHeader";