userChrome.css for a more compact Firefox UI

/*    Menu Bar height    */
#toolbar-menubar {
  margin-top: 0px !important;
  margin-bottom: 0px !important;
  padding-top: 0px !important;
  padding-bottom: 0px !important;
  line-height: 22px !important;
  max-height: 22px !important;
}
/* Fixing title bar buttons (close/min/max) due to shortened Menu Bar height */
#toolbar-menubar .titlebar-button {
  padding-block: 1px !important;
}

/*    Tabs/Tab Bar height    */
:root {
 --tab-min-height: 20px !important;
 --tab-max-height: 20px !important;
}
#TabsToolbar {
  max-height: 25px !important;
}

/*    Toolbar/Address bar/URL bar height    */
/* partially based on https://github.com/CarterSnich/firefox-xtra-compact/blob/master/chrome/userChrome.css */
toolbar#nav-bar {
  height: 25px !important;
}
.urlbar-input-container {
  height: 24px !important;
  margin-top: 4px !important;
}
#urlbar-background {
  height: 21px !important;
  margin-top: 5px !important;
}
/* centering the url text line vertically */
#urlbar-input {
  padding-bottom: 2px !important;
}

/* removing the empty spaces in the toolbar sides */
#back-button {
  padding-left: 0px !important;
}
#PanelUI-menu-button {
  padding-right: 0px !important;
}

/*    Menu Items height/padding    */
menupopup > menu:not(bookmarksPopup), /* this is for the lines of the sub-menus/folders/containers */
menupopup > menuitem:not(bookmarksPopup) {
  padding-block: 3px !important; /* above and below each item */
  margin-left: 0px !important; /* margin is the outer space around an item */
  padding-left: 8px !important; /* padding is the inner space inside an item */
  margin-right: 0px !important;
  padding-right: 0px !important;
}

/* the padding of the menu itself */
menupopup {
  --panel-padding: 1px !important;
}

/* the text part of the item/sub-menu */
menupopup > menu > hbox, /* this is the text part of the sub-menus/folders/containers */
menupopup > menuitem > hbox {
  margin-left: 0px !important;
  margin-right: 4px !important; /* not 0px because otherwise the text appears too close to the favicon */
}

/* removing padding for separators */
menuseparator {
  padding-block: 0px !important;
}

/* padding for the "hamburger menu" and extensions menu in the toolbar */
/* the values in the first line refer to the top-bottom and left-right padding respectively */
:root{
--arrowpanel-menuitem-padding: 1px 1px !important;
--arrowpanel-menuitem-margin-inline: 1px !important;
--arrowpanel-menuitem-padding-block: 1px !important;
--panel-separator-margin-horizontal: 1px !important;
--panel-subview-body-padding-block: 1px !important;
}

/* height/placement of the "go" arrow when typing/searching in the urlbar */
#urlbar-go-button {
  padding-top: 0px !important;
  padding-bottom: 0px !important;
  line-height: 20px !important;
  max-height: 20px !important;
}

/* padding of the left-side icons in the url bar */
#tracking-protection-icon-container, #identity-icon-box, #identity-permission-box {
  padding: 4px !important;
}

/* spacing/padding around addons icons in the toolbar/urlbar */
:root {
  --toolbarbutton-outer-padding: 0px !important;
  --toolbarbutton-inner-padding: 5px !important;
}
.toolbaritem-combined-buttons:not([widget-type='button-and-view']),
.toolbaritem-menu-buttons {
  margin-inline: 0px !important;
  margin-inline-start: 0px !important;
  margin-inline-end: 0px !important;
}

/* padding of the New Tab button */
#tabs-newtab-button {
  padding-left: 3px !important;
}