"Tabs on Top" is a visible option if you had it disabled.

"Tabs on Top" will not be visible if it was left on its default setting, enabled. You can toggle it by setting the browser.tabs.onTop preference in about:config.

• http://kb.mozillazine.org/About:config

• How to make the new Firefox look like the old Firefox

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.

You need to close and restart Firefox to see the menu entry appear or disappear.