mobile-config-firefox/chrome/hide_tabs_with_one_tab.css

/* Makes tabs toolbar items zero-height initially and sets enlarge them to fill up space equal to tab-min-height set on tabs */
/* Firefox 65+ only */

/* REMOVE ALL BUTTONS you can from the tabs toolbar or this won't work properly */

:root[sizemode="normal"] #nav-bar{ --uc-window-drag-space-width: 20px }

#titlebar{ -moz-appearance: none !important; }

#tabbrowser-tabs, #tabbrowser-tabs > .tabbrowser-arrowscrollbox{ min-height: 0 !important; }

:root:not([customizing]) #tabbrowser-tabs .tabs-newtab-button,
:root:not([customizing]) #tabs-newtab-button,
:root:not([customizing]) #TabsToolbar .titlebar-button{
  -moz-appearance: none !important;
  height: 0px;
  padding-top: 0px !important;
  padding-bottom: 0px !important;
  -moz-box-align: stretch;
  margin: 0 !important;
}
#tabbrowser-tabs .tabbrowser-tab{ height: var(--tab-min-height) }

#tabbrowser-tabs .tabbrowser-tab[first-visible-tab="true"][last-visible-tab="true"]{
  visibility: collapse !important;
}

/* Button re-styling */
#tabs-newtab-button:hover,
#tabbrowser-tabs .tabs-newtab-button:hover{ background-color: var(--toolbarbutton-hover-background) }

#tabs-newtab-button > .toolbarbutton-icon,
#tabbrowser-tabs .tabs-newtab-button > .toolbarbutton-icon{
  padding: 0 !important;
  transform: scale(0.6);
  background-color: transparent !important;
}
/* Extra top padding  in maximized window */
:root[sizemode="maximized"] > #navigator-toolbox{ padding-top:7px !important; }

/* Fix window controls not being clickable */
#toolbar-menubar:hover{
  min-height: calc(var(--tab-min-height) + var(--space-above-tabbar) - 1px) !important;
  height: calc(var(--tab-min-height) + var(--space-above-tabbar) - 1px) !important;
  -moz-appearance: initial !important;
}
#nav-bar::before,
#nav-bar::after{
  content: "";
  display: -moz-box;
  width: var(--uc-window-drag-space-width);
}