I think this might be a bug in FireFox. If the TreePanel has nodes that extend beyond the panel bounds and scrollbars appear, when hovering over the nodes and then out to the center panel, the hover effect remains. This does not seem to be a problem in IE. here is the code to reproduce it:

    <ext:ViewPort ID="ViewPort1" runat="server">
            <ext:FitLayout ID="FitLayout1" runat="server">
                <ext:Panel runat="server" Height="300" Title="" Border="false">
                        <ext:BorderLayout ID="BorderLayout1" runat="server">
                            <West Collapsible="false" Split="true">
                                <ext:TreePanel ID="tvwFolders" runat="server" Title="Project Folders" AutoScroll="true" Animate="false" Border="true" TrackMouseOver="true" Width="300">
                                        <BeforeLoad Fn="nodeLoad" />
                                <ext:Panel ID="Panel2" runat="server" Title="Center">