userChrome.css

/* Hide sidebar header, when using Tree Style Tab. */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
    visibility: collapse;
}
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
  opacity: 0;
  pointer-events: none;
}
#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
    visibility: collapse !important;
}

/* --- Disable URL bar expanding --- */

#urlbar[breakout][breakout-extend] {
  top: calc((var(--urlbar-toolbar-height) - var(--urlbar-height)) / 2) !important;
  left: 0 !important;
  width: 100% !important;
}

#urlbar[breakout][breakout-extend] > #urlbar-input-container {
  height: var(--urlbar-height) !important;
  padding-block: 0px !important;
  padding-inline: 0px !important;
}

#urlbar[breakout][breakout-extend][breakout-extend-animate] > #urlbar-background {
  animation-name: none !important;
}

/* Hide sidebar header, when using Tree Style Tab. */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
    visibility: collapse;
}

/* Sidebar min and max width removal */
#sidebar {
    max-width: none !important;
    min-width: 0px !important;
}