userChrome.css

/*Command + L on Mac to focus on address bar*/
#nav-bar {
  /* customize this value. */
  --navbar-margin: -44px;

  margin-top: var(--navbar-margin);
  margin-bottom: 0;
  z-index: -100;
  transition: all 0.3s ease !important;
  opacity: 0;
}

#navigator-toolbox:focus-within > #nav-bar,
#navigator-toolbox:hover > #nav-bar
{
  margin-top: 0;
  margin-bottom: var(--navbar-margin);
  z-index: 100;
  opacity: 1;
}

/*
 *    Hide Tabs on Topbar
 */
#TabsToolbar {
    visibility: collapse;
}

/* Hide main tabs toolbar */
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
    opacity: 0;
    pointer-events: none;
}
#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
    visibility: collapse !important;
}

/* Sidebar min and max width removal */
#sidebar {
    max-width: none !important;
    min-width: 0px !important;
}
/* Hide splitter, when using Tree Style Tab. */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] + #sidebar-splitter {
    display: none !important;
}
/* Hide sidebar header, when using Tree Style Tab. */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
    visibility: collapse;
}

/* Shrink sidebar until hovered, when using Tree Style Tab. */
:root {
    --thin-tab-width: 30px;
    --wide-tab-width: 200px;
}
#sidebar-box:not([sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"]) {
    min-width: var(--wide-tab-width) !important;
    max-width: none !important;
}
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] {
    position: relative !important;
    transition: all 100ms !important;
    min-width: var(--thin-tab-width) !important;
    max-width: var(--thin-tab-width) !important;
}
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"]:hover {
    transition: all 100ms !important;
    min-width: var(--wide-tab-width) !important;
    max-width: var(--wide-tab-width) !important;
    margin-right: calc((var(--wide-tab-width) - var(--thin-tab-width)) * -1) !important;
    z-index: 1;
}