Navibar is a very interesting Firefox extension. When you have it installed, Navibar will automatically check every site you visit for a sitemap.rdf file (In fact it was the requests for sitemap.rdf in my server logs which pointed me towards this extension). If it finds a sitemap.rdf file, it will display a structure of the site in the sidebar like this.
A RDF format is used to publish sitemaps or an XML format based on Google Sitemaps. The fact that barely any sites publish sitemaps and a standard Google Sitemap is useless to this extension (for a start, there is no standard location for a Google Sitemap) severely limits the usefulness of this extension.
Even then, a navigation sidebar doesn’t really make it that much easier to navigate the site. It’s great because it stays out of the way and you can configure the extension to automatically show the sidebar when it finds a sitemap and hide it when it can’t find a sitemap.
If you use Firefox on an internal intranet Navibar can be great.