userChrome2.css

/* Makes tabs to appear on multiple lines below the web content */
/* Tab reordering will not work and can't be made to work */

/* Make menubar enabled for getting window controls in correct place. Menubar options will be shown as overlay when mouse is at top-left corner */

/* It's recommended to move tabs new-tab-button outside tabs toolbar */

/* Change the --multirow-n-rows to change maximum number of rows before the rows will start to scroll  */
/* Scrollbar can't be clicked but the rows can be scrolled with mouse */
/* This maximum visible rows won't work before Fx66 */
/* So this setting does nothing on Fx65 and all tab rows will be shown */

:root{
  --multirow-toolbar-height: 0px;
  --multirow-top-padding: 0px;
  --multirow-n-rows: 3;
}

:root[sizemode="maximized"]{ --multirow-top-padding: 0px }
:root[uidensity="compact"]{--multirow-toolbar-height: 0px}
:root{ border-top-width: 0px !important }

#main-window > body > box,
#navigator-toolbox{ -moz-box-ordinal-group: 2 }

#titlebar{-moz-appearance: none !important; } /* Try setting to "-moz-window-titlebar" if you face issues */
/* Fix incorrect vertical alignment with megabar */
#urlbar[breakout]{ --urlbar-toolbar-height: var(--multirow-toolbar-height) !important; }

#customization-container,
:root:not([inDOMFullscreen]) #content-deck,
:root:not([inDOMFullscreen]) #browser{ margin-top: calc(var(--multirow-toolbar-height) + var(--multirow-top-padding))}

/* Restyle nav-bar and menubar */
#nav-bar-customization-target{ width: calc(100% - 100px); }
:root[uidensity="compact"] #nav-bar-customization-target{ width: calc(100% - 82px); }

#nav-bar-overflow-button,
#PanelUI-menu-button{ height: var(--multirow-toolbar-height); }

#toolbar-menubar{
  height: var(--multirow-toolbar-height);
  position: fixed;
  display: flex;
  top: var(--multirow-top-padding);
  width: 100vw;
  color: var(--lwt-text-color);
  background: linear-gradient(to right, var(--toolbar-bgcolor), var(--toolbar-bgcolor) 30px, transparent 30px) no-repeat;
}
#menubar-items + spacer{ flex-grow: 1 }

#toolbar-menubar > #menubar-items{
  -moz-box-pack: center;
  height: 100%;
  visibility: hidden;
}

#toolbar-menubar:hover{ z-index: 2 }
#toolbar-menubar:hover > #menubar-items{
  visibility: visible;
  background-image: linear-gradient( to left,transparent,var(--lwt-accent-color) 35px);
}

#toolbar-menubar > .titlebar-buttonbox-container{ display: block; float: right; height: 100%; background: var(--toolbar-bgcolor)}
#toolbar-menubar > .titlebar-buttonbox-container > .titlebar-buttonbox{ height: 100% ;}

#main-menubar:last-child{ padding-inline-end: 40px }
#toolbar-menubar > toolbarspring {
  background: url(chrome://browser/skin/settings.svg) no-repeat 10px;
  height: 100%;
  display: block;
  fill: currentColor;
  -moz-context-properties: fill;
  min-width: 28px !important;
  max-width: 28px !important;
}

#PanelUI-button{ margin-right: 20px; }

#tabbrowser-tabs{
  min-height: unset !important;
}
#tabbrowser-tabs > .tabbrowser-arrowscrollbox > .arrowscrollbox-scrollbox,
#tabbrowser-arrowscrollbox{
  display: flex;
  flex-wrap: wrap;
  max-height: calc(var(--multirow-toolbar-height) * var(--multirow-n-rows));
  overflow-y: auto;
  scrollbar-color: var(--toolbar-bgcolor) var(--lwt-accent-color);
  scrollbar-width: thin;
}
#tabbrowser-tabs > .tabbrowser-arrowscrollbox {
  overflow: -moz-hidden-unscrollable;
  display: block;
}

/* Selectors for Firefox 71+ */
/* These are not tabs toolbar specific but horizontal scrollbox isn't used elsewhere, except in bookmarks toolbar but there it doesn't have [part] attribute since it's not in shadow-root */
@-moz-document url(chrome://browser/content/browser.xhtml){
  .scrollbutton-up[orient="horizontal"][part]~spacer,
  .scrollbutton-up[orient="horizontal"][part],
  .scrollbutton-down[orient="horizontal"][part]{ display: none }

  scrollbox[part][orient="horizontal"]{
    display: flex;
    flex-wrap: wrap;
    overflow-y: auto;
    max-height: calc(var(--tab-min-height) * var(--multirow-n-rows));
    scrollbar-color: currentColor transparent;
    scrollbar-width: thin;
  }
  #tabbrowser-tabs{ padding-inline-start: 0px !important; }
}

.tabbrowser-tab{ max-height: var(--tab-min-height); min-height: var(--tab-min-height) }
.tabbrowser-tab > stack{ width: 100%; height: 100%; }

.tabbrowser-tab[fadein]:not([pinned]){
  min-width: 100px !important;
  flex-grow: 1;
  /*
  Uncomment to enable full-width tabs, also makes tab dragging a tiny bit more sensible
  Don't set to none or you'll see errors in console when closing tabs
  */
  /* max-width: 100vw !important; */
}

.tabbrowser-tab[pinned]{
  position: static !important;
  margin-inline-start: 0px !important;
}

#scrollbutton-up,
#scrollbutton-down,
#scrollbutton-up~spacer,
#tabbrowser-tabs .scrollbutton-up,
#tabbrowser-tabs .scrollbutton-down,
#alltabs-button,
#TabsToolbar #new-tab-button,
#tabbrowser-tabs spacer,
.tabbrowser-tab::after{ display: none !important } /* Also disables tab separators since they mysteriously break tab-row scrolling */

/* Always show tab close button on hover and never otherwise */
.tabbrowser-tab .tab-close-button{
  display:none;
}
.tabbrowser-tab:not([pinned]):hover .tab-close-button{
  display:-moz-box !important;
}