The Menu bar is a special toolbar. As a toolbar, you can dock and undock it, and customise it (for example you could remove a menu or add a new menu).
Unlike the other toolbars however:
You cannot hide the Menu bar.
For menu buttons within the Menu bar there is more limited customisation available than for other menu buttons; you can only change the button's text.
It is re-set using a different method to other toolbars.