userChrome.css

/* Linux & macOS specific styles */
@media (-moz-gtk-csd-available), (-moz-mac-big-sur-theme: 0), (-moz-mac-big-sur-theme: 1) {
    #TabsToolbar:not([customizing="true"]) {
          visibility: collapse !important;
      }

    #toolbar-menubar {
          padding-top: 0px !important;
      }

    #TabsToolbar:not([customizing="true"]) {
          visibility: collapse !important;
      }

    #toolbar-menubar {
          padding-top: 0px !important;
      }

    #nav-bar {
          padding-left: 80px;
      }

    #TabsToolbar .titlebar-buttonbox-container {
          visibility: visible !important;
          position: absolute;
          top: 12px;
          left: 0px;
      }

    .titlebar-buttonbox-container {
          display: block;
      }

    .titlebar-button > .toolbarbutton-icon {
          height: 13x !important;
          min-height: 13px !important;
          width: 13px !important;
          min-width: 13px !important;
      }

    .titlebar-button {
          padding-left: 6px !important;
          padding-right: 6px !important;
          padding-top: 6px !important;
      }

    .titlebar-placeholder[type="pre-tabs"] {
          width: 120px !important;
      }

    .titlebar-buttonbox-container {
          -moz-box-ordinal-group: 0;
      }
}

/* Windows specific styles */
@media (-moz-os-version: windows-win10) { 
    /* Hide main tabs toolbar */
    :root[tabsintitlebar]{
          --uc-window-control-width: 138px; /* Space at the right of nav-bar for window controls */
          /* --uc-window-drag-space-width: 24px; */  /* To add extra window drag space in nav-bar */
      }

    #nav-bar{
          border-inline: var(--uc-window-drag-space-width,0px) solid var(--toolbar-bgcolor) ;
          border-inline-style: solid !important;
          border-right-width: calc(var(--uc-window-control-width,0px) + var(--uc-window-drag-space-width,0px));

          padding-top: 0px !important;
      }

    :root {
          --uc-toolbar-height: 32px;
          --chrome-content-separator-color: none !important;
      }

    :root:not([uidensity="compact"]) {
          --uc-toolbar-height: 38px;
      }

    #TabsToolbar {
          visibility: collapse !important;
      }

    :root:not([inFullscreen]) #nav-bar {
          margin-top: calc(0px - var(--uc-toolbar-height));
      }

    #toolbar-menubar {
          min-height: unset !important;
          height: var(--uc-toolbar-height) !important;
          position: relative;
      }

    #main-menubar {
          -moz-box-flex: 1;
          background-color: var(--toolbar-bgcolor,--toolbar-non-lwt-bgcolor);
          background-clip: padding-box;
          border-right: 30px solid transparent;
          border-image: linear-gradient(to left, transparent, var(--toolbar-bgcolor,--toolbar-non-lwt-bgcolor) 30px) 20 / 30px
      }

    #toolbar-menubar:not([inactive]) {
          z-index: 2;
      }

    #toolbar-menubar[inactive] > #menubar-items {
          opacity: 0;
          pointer-events: none;
          margin-left: var(--uc-window-drag-space-width,0px);
      }


    :root[inFullscreen] #nav-bar {
          border-inline: none !important;
      }
}

/* General styles */
#sidebar-box #sidebar-header {
    visibility: collapse;
    display: none;
}

#sidebar-box:not([hidden]) {
    display: block;
    position: fixed;
    min-width: 48px;
    max-width: 48px;
    overflow: hidden;
    transition: all 0.2s ease;
    border-right: 1px solid var(--sidebar-border-color);
    z-index: 1;
}

#sidebar,
#sidebar-box:hover {
    min-width: 20vw !important;
    max-width: 20vw !important;
}

@media (width >= 1200px) {
    #sidebar,
    #sidebar-box:hover {
          min-width: 10vw !important;
          max-width: 10vw !important;
      }
}

#sidebar-splitter {
    display: none;
}

#sidebar {
    height: calc(100vh - 32px);
}

#sidebar-box:not([hidden]) ~ #appcontent {
    margin-left: 48px;
}

#main-window[inFullscreen][inDOMFullscreen] #appcontent {
    margin-left: 0;
}

#main-window[inFullscreen] #sidebar {
    height: 100vh;
}

#sidebar-header {
    background: #0C0C0D;
    border-bottom: none !important;
}

#sidebar-splitter {
    border-right-color: #0C0C0D !important;
    border-left-color: #0C0C0D !important;
}

#sidebar-switcher-target,
#sidebar-close {
    filter: invert(100%);
}