Re: How can we disable address bar (navigation bar) but should display multiple tabs?
Does it help if you move the tab bar to below the toolbars (i.e. not on top)?
Current Firefox versions only show the "Tabs on Top" menu entry in the "View > Toolbars" and "Firefox > Options" menu and in toolbar context menus if the tabs aren't in the default position on top.
If the tabs are on top and the menu entry isn't available and you want to move the tabs below the navigation toolbar then you need to flip the browser.tabs.onTop pref to false on the about:config page.
Note that even if you hide the Navigation Toolbar it is still possible to bring up a location bar via the Ctrl+L shortcut.