It looks like there is a problem with skinning native .net dropdownlist in Firefox 4 beta 7. I realize this is beta, but wanted to bring it to your attention before it went to production. Sample project available, aspx page copied below.. Works fine in IE8 and older versions of Firefox. Skinnedddl.jpg is screenshot from Firefox.
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="_Default" %>
<%@ Register Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" TagPrefix="telerik" %>
<%@ Register Assembly="System.Web.DataVisualization, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
Namespace="System.Web.UI.DataVisualization.Charting" TagPrefix="asp" %>
<%@ Register Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" TagPrefix="telerik" %>
<%--<%@ Register src="UserControls/navtabs.ascx" tagname="NavTabs" tagprefix="nt" %>
<%@ Register src="UserControls/navheader.ascx" tagname="NavHead" tagprefix="nh" %>--%>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<%@ Register Assembly="System.Web.Ajax" Namespace="System.Web.UI" TagPrefix="asp" %>
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
</div>
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" EnablePartialRendering="true" CombineScripts="false"> </asp:ToolkitScriptManager>
<telerik:RadFormDecorator runat="server" ID="RadFormDecorator1" EnableEmbeddedSkins="true" DecoratedControls="All"/>
<asp:DropDownList runat="server" ID="ddlArea" AutoPostBack="false" Width="200" >
<asp:ListItem Value="-1" Text="Select County"></asp:ListItem>
<asp:ListItem Value="26" Text="Baker"></asp:ListItem>
<asp:ListItem Value="1" Text="Benton"></asp:ListItem>
<asp:ListItem Value="2" Text="Clackamas"></asp:ListItem>
<asp:ListItem Value="3" Text="Clark"></asp:ListItem>
<asp:ListItem Value="4" Text="Clatsop"></asp:ListItem>
<asp:ListItem Value="5" Text="Columbia"></asp:ListItem>
<asp:ListItem Value="6" Text="Coos"></asp:ListItem>
<asp:ListItem Value="7" Text="Cowlitz"></asp:ListItem>
<asp:ListItem Value="27" Text="Crook"></asp:ListItem>
<asp:ListItem Value="28" Text="Curry"></asp:ListItem>
<asp:ListItem Value="8" Text="Deschutes"></asp:ListItem>
<asp:ListItem Value="9" Text="Douglas"></asp:ListItem>
<asp:ListItem Value="29" Text="Gilliam"></asp:ListItem>
<asp:ListItem Value="30" Text="Grant"></asp:ListItem>
<asp:ListItem Value="31" Text="Harney"></asp:ListItem>
<asp:ListItem Value="10" Text="Hood River"></asp:ListItem>
<asp:ListItem Value="11" Text="Jackson"></asp:ListItem>
<asp:ListItem Value="12" Text="Jefferson"></asp:ListItem>
<asp:ListItem Value="13" Text="Josephine"></asp:ListItem>
<asp:ListItem Value="14" Text="Klamath"></asp:ListItem>
<asp:ListItem Value="32" Text="Lake"></asp:ListItem>
<asp:ListItem Value="15" Text="Lane"></asp:ListItem>
<asp:ListItem Value="16" Text="Lane (coastal)"></asp:ListItem>
<asp:ListItem Value="17" Text="Lincoln"></asp:ListItem>
<asp:ListItem Value="18" Text="Linn"></asp:ListItem>
<asp:ListItem Value="33" Text="Malheur"></asp:ListItem>
<asp:ListItem Value="19" Text="Marion"></asp:ListItem>
<asp:ListItem Value="34" Text="Morrow"></asp:ListItem>
<asp:ListItem Value="20" Text="Multnomah"></asp:ListItem>
<asp:ListItem Value="21" Text="Polk"></asp:ListItem>
<asp:ListItem Value="35" Text="Sherman"></asp:ListItem>
<asp:ListItem Value="22" Text="Tillamook"></asp:ListItem>
<asp:ListItem Value="23" Text="Umatilla"></asp:ListItem>
<asp:ListItem Value="36" Text="Union"></asp:ListItem>
<asp:ListItem Value="37" Text="Wallowa"></asp:ListItem>
<asp:ListItem Value="38" Text="Wasco"></asp:ListItem>
<asp:ListItem Value="24" Text="Washington"></asp:ListItem>
<asp:ListItem Value="39" Text="Wheeler"></asp:ListItem>
<asp:ListItem Value="25" Text="Yamhill"></asp:ListItem>
</asp:DropDownList>
<asp:CustomValidator runat="server" Font-Size="8" id="CustomValidator6" ControlToValidate="ddlArea" ErrorMessage="<br> * Area is required." Display="Dynamic" />
<telerik:RadToolTip ID="txtCountyTT" runat="server" Width="150" AutoCloseDelay="10000"
TargetControlID="ddlArea" Position="MiddleRight" OffsetY="0" OffsetX="10" ShowDelay="300"
Text="Required: County where most employees represented by this survey work.">
</telerik:RadToolTip>
</form>
</body>
</html>