Moves the menu bar bellow the address bar in Firefox.

menubar.uc.js

// ==UserScript==
// @name            menubar on bottom
// @include         main
// @author          valinet
// ==/UserScript==

UC.menubarbottom = {
  
  resize: function() {
	var elem = document.getElementById("TabsToolbar").getElementsByClassName("toolbar-items")[0];
	if (window.windowState == 1) // 1 = maximized, 3 = normal
	{
	  elem.style = "";
	}
	else
	{
	  elem.style = "padding-top: var(--space-above-tabbar)";
	}
  },
  
  init: function () {
	  
	let menubar = document.getElementById("toolbar-menubar");
	let tbParent = document.getElementById("navigator-toolbox");
	
	// hide Min, Max, Close buttons on the menu bar
	menubar.getElementsByClassName("titlebar-buttonbox-container")[0].style = "display: none";
	
	// style menu bar with semi-transparent overlay (same as address bar)
	menubar.style = "background-color: var(--toolbar-bgcolor);";
	
	// move menu bar bellow the address bar
	tbParent.appendChild(menubar);
	
	// add/remove top padding (drag space) according to window being normal/maximized
	// https://developer.mozilla.org/en-US/docs/Archive/Mozilla/XUL/Events#Window_events
	UC.menubarbottom.resize();
	window.addEventListener('sizemodechange', UC.menubarbottom.resize);
	
	// move bookmarks bar bellow menu bar
	tbParent.appendChild(document.getElementById("PersonalToolbar"));
  }
}

UC.menubarbottom.init();