In Firefox 18.0.1 on two different machines: one lets me select (via right click menu) the Tab Bar on top, the other does not show this selection?
The machine that has the option for tabs on top: Win XP Pro 2002 SP3
The machine that does not have the option for tabs on top: Win XP Pro 2002 SP2
Wšykne wótegrona (2)
"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.
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.