userChrome.css

/* Firefox Ultra Compact Mode
 *
 * Copyright (c) Danny Colin
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/.
 */

:root {
  /* Appmenu: Reduce item padding */
  --arrowpanel-menuitem-padding-block: 4px 8px !important;
  /* Tabbar: reduce tab margin */
  --tab-block-margin: 4px 3px !important;
}

/* Tab: Reduce height */
.tabbrowser-tab {
  min-height: 24px !important;
}

/* Tab: Ensure tab height doesn't augment when arrowscrollbox is visible  */ 
#tabbrowser-arrowscrollbox {
  --tab-min-height: 31px !important;
  max-height: var(--tab-min-height);
}

/* URLBar: Fix vertical alignment */
#urlbar[breakout=true]:not([open="true"]) {
	--urlbar-height: 20px !important;
	--urlbar-toolbar-height: 24px !important;
}

/* URLBar: Fix URL address vertical aligment when megabar is open */
#urlbar[breakout=true][open="true"] {
	--urlbar-toolbar-height: 30px !important;
}

/* Searchbar: Ensure toolbar height doesn't augment when searchbar is visible */
#urlbar-container, 
#search-container {
	padding-block: 0 !important;
}

/* Searchbar: Make sure the min-height of the input is the same as the popup */
#search-container {
  min-width: 192px !important;
}

/* Toolbar: Reduce spacing */
#urlbar-container {
	--urlbar-container-height: 30px !important;
  margin-top: 0 !important;
}

/* Reload Button: Fix vertical alignment */
#reload-button {
	margin-block-start: -2px !important;
}

/* Windows 10 context menu */
@media (-moz-os-version: windows-10) {
  /* Context Menu: Reduce vertical space */
  menupopup > menuitem,
  menupopup > menu {
    padding-block: 2px !important;
  }
}