userChrome.css @MacBook

userChrome.css

Using the side-bar with Tree tab mode
Auto-hide
No sidebar-header */

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

#main-window {
    --toolbar-bgcolor: #31363b !important;
}

#tabbrowser-tabs {
    visibility: collapse !important;
}
#sidebar-header {
  visibility: collapse !important;
}

.closebox {
  display: none
}

.tab:hover .closebox {
  display: block;
}

.tab {
  padding-top: 0px;
  padding-bottom: 0px;
}

.tab .icon {
 padding-top: 2px; /* reducing it by 2px */
 padding-bottom: 2px;
}

:root {
  --tab-height: 20px; /* reducing it by 4px */
  --sidebar-hover-width: 15px;
  --sidebar-visible-width: 260px;
}

/* #sidebar-splitter styles the divider between the sidebar and the rest of the browser. */
#sidebar-splitter {
}