Open the Core database, navigate to /sitecore/layout/Devices/Internet Explorer and delete the Firebug Lite item.
This information originally comes from a Sitecore developer.
<siteMap defaultProvider="CurrentNavigation" enabled="true">
<providers>
<add name="CustomTopNavProvider" siteMapFile="/_layouts/1033/CustomTopNavMenu.sitemap" type="System.Web.XmlSiteMapProvider,System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
Add the CustomTopNavMenu.sitemap to the 1033 directory. If you don't know how to create or use a sitemap file just google it.
Open up the master page for each site collection, for 2010 it's v4, probably best to create a copy and all that stuff before editing it.
Find this part of the file. (around line 343)
<SharePoint:AspMenu
ID="TopNavigationMenuV4"
Runat="server"
EnableViewState="false"
DataSourceID="topSiteMap"
AccessKey="<%$Resources:wss,navigation_accesskey%>"
UseSimpleRendering="true"
UseSeparateCss="false"
Orientation="Horizontal"
StaticDisplayLevels="2"
MaximumDynamicDisplayLevels="1"
SkipLinkText=""
CssClass="s4-tn"/>
<SharePoint:DelegateControl runat="server" ControlId="TopNavigationDataSource" Id="topNavigationDelegate">
<Template_Controls>
<asp:SiteMapDataSource
ShowStartingNode="False"
SiteMapProvider="SPNavigationProvider"
id="topSiteMap"
runat="server"
StartingNodeUrl="sid:1002"/>
</Template_Controls>
</SharePoint:DelegateControl>
<SharePoint:AspMenu
ID="TopNavigationMenuV4"
Runat="server"
EnableViewState="false"
DataSourceID="topSiteMap"
AccessKey="<%$Resources:wss,navigation_accesskey%>"
UseSimpleRendering="true"
UseSeparateCss="false"
Orientation="Horizontal"
StaticDisplayLevels="1"
MaximumDynamicDisplayLevels="4"
SkipLinkText=""
CssClass="s4-tn"/>
<SharePoint:DelegateControl runat="server" ControlId="TopNavigationDataSource">
<Template_Controls>
<asp:SiteMapDataSource
ShowStartingNode="False"
SiteMapProvider="CustomTopNavProvider"
id="topSiteMap"
runat="server"
/>
</Template_Controls>
</SharePoint:DelegateControl>
That's it.