fenix-fox/chrome/hide_tabs_with_one_tab.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/hide_tabs_with_one_tab.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* Makes tabs toolbar items zero-height initially and sets enlarge them to fill up space equal to tab-min-height set on tabs. Optionally use privatemode_indicator_as_menu_button.css to replace main menu icon with private browsing indicator while tabs are hidden. */
/* Firefox 65+ only */

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

#titlebar{ -moz-appearance: none !important; }
#TabsToolbar{ min-height: 0px !important }

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

:root:not([customizing]) #tabbrowser-tabs .tabs-newtab-button,
:root:not([customizing]) #tabs-newtab-button,
:root:not([customizing]) #TabsToolbar-customization-target > .toolbarbutton-1,
: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;
}

.accessibility-indicator,
.private-browsing-indicator{ 
  height: unset !important;
}
.accessibility-indicator > hbox{ padding-block: 0 !important }

.tabbrowser-tab{
  height: calc(var(--tab-min-height) + 2 * var(--tab-block-margin,0px));
}

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

/* Button re-styling */
#tabs-newtab-button{ transform: scale(0.8); }
#tabs-newtab-button:hover{
  background-color: var(--toolbarbutton-hover-background);
  border-radius: var(--tab-border-radius);
}

#tabs-newtab-button > .toolbarbutton-icon{
  padding: 0 !important;
  transform: scale(0.6);
  background-color: transparent !important;
}
/* Extra top padding  in maximized window */
@media  (-moz-platform: windows),
        (-moz-os-version: windows-win7),
        (-moz-os-version: windows-win10){
  :root[tabsintitlebar][sizemode="maximized"] #navigator-toolbox{
    padding-top: var(--uc-titlebar-padding,7px);
  }
}
/* Fix window controls not being clickable */
:root[tabsintitlebar] #toolbar-menubar[autohide="true"][inactive]{
  transition: height 0ms steps(1) 80ms;
}
:where(#nav-bar){
  border-inline: var(--uc-window-drag-space-width,0) solid var(--toolbar-bgcolor);
}