The tab sidebar extension, if you haven't heard of it already, provides a sidebar in the browser that can act as a replacement to the main tab bar, with extras. The most obvious extra is that each tab includes a preview of the web page that is continually updated, but you also get navigation buttons for each tab.

Since Tab Sidebar provides a replacement for the main tab bar, when the sidebar is visible, Firefox's main tab bar will be hidden. If you still wish to use the main tab bar then you can show it again from the Tab Sidebar options.