firefox-csshacks/chrome/autohide_bookmarks_and_main_toolbars.css

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

#navigator-toolbox{
  --uc-bm-padding: 4px; /* Vertical padding to be applied to bookmarks */
  --uc-bm-height: calc(20px + 2 * var(--uc-bm-padding)); /* Might need to adjust if the toolbar has other buttons */
  --uc-navbar-height: -40px; /* navbar is main toolbar. Use negative value */
  --uc-autohide-toolbar-delay: 600ms; /* The toolbar is hidden after 0.6s */
}

:root[uidensity="compact"] #navigator-toolbox{
  --uc-navbar-height: -34px;
}
:root[uidensity="touch"] #navigator-toolbox{ --uc-bm-padding: 6px }

:root[chromehidden~="directories"] #navigator-toolbox{
  --uc-bm-height: 0px;
}
:root[chromehidden~="toolbar"] #navigator-toolbox{
  --uc-navbar-height: 0px;
}
#navigator-toolbox,
#sidebar-box,
#sidebar-main,
#sidebar-splitter,
#tabbrowser-tabbox{
  z-index: auto !important;
}

:root[sessionrestored] #nav-bar,
:root[sessionrestored] #PersonalToolbar{
  background-image: linear-gradient(var(--toolbar-bgcolor),var(--toolbar-bgcolor)), var(--lwt-additional-images,var(--toolbar-bgimage))  !important;
  background-position: top,var(--lwt-background-alignment);
  background-position-y: calc(0px - var(--tab-min-height) - 2*var(--tab-block-margin,0px));
  background-repeat: repeat,var(--lwt-background-tiling);
  transform: rotateX(90deg);
  transform-origin: top;
  transition: transform 135ms linear var(--uc-autohide-toolbar-delay) !important;
  z-index: 2;
}
#PlacesToolbarItems > .bookmark-item,
#OtherBookmarks,
#PersonalToolbar > #import-button{
  padding-block: var(--uc-bm-padding) !important;
}
:root[sessionrestored] #PersonalToolbar{
  z-index: 1;
  background-position-y: calc(0px - var(--tab-min-height) - 2*var(--tab-block-margin,0px) + var( --uc-navbar-height));
}

:root[lwtheme-image] #nav-bar,
:root[lwtheme-image] #PersonalToolbar{
  background-image: linear-gradient(var(--toolbar-bgcolor),var(--toolbar-bgcolor)),var(--lwt-header-image), var(--lwt-additional-images,var(--toolbar-bgimage)) !important;
}

#nav-bar[customizing],#PersonalToolbar[customizing]{ transform: none !important }

#navigator-toolbox > #PersonalToolbar{
  transform-origin: 0px var(--uc-navbar-height);
  position: relative;
}
:root[sessionrestored] #urlbar[popover]{
  opacity: 0;
  pointer-events: none;
  transition: transform 135ms linear var(--uc-autohide-toolbar-delay), opacity 0ms calc(var(--uc-autohide-toolbar-delay) + 135ms);
  transform-origin: 0px calc((var(--urlbar-container-height) - var(--urlbar-height)) / 2);
  transform: rotateX(89.5deg);
}
#mainPopupSet:has(> [panelopen]:not(#ask-chat-shortcuts,#tab-preview-panel)) ~ toolbox #urlbar[popover],
#navigator-toolbox:is(:hover,:focus-within) #urlbar[popover],
#urlbar-container > #urlbar[popover]:is([focused],[open]){
  opacity: 1;
  pointer-events: auto;
  transition-delay: 100ms;
  transform: rotateX(0deg);
}

:root[sessionrestored]:not([customizing]) #navigator-toolbox{
  margin-bottom:  calc(-1px - var(--uc-bm-height) + var(--uc-navbar-height));
}

/* Make sure the bookmarks toolbar is never collapsed even if it is disabled */
:root[sizemode="fullscreen"] #PersonalToolbar,
#PersonalToolbar[collapsed="true"]{
  min-height: initial !important;
  max-height: initial !important;
  visibility: hidden !important
}
#PersonalToolbar[collapsed="true"] #PlacesToolbarItems > *,
:root[sizemode="fullscreen"] #PersonalToolbar #PlacesToolbarItems > *{
  visibility: hidden !important;
}

/* If bookmarks toolbar is collapsed on startup, then no items are generated, and we need to set some height for it */
#PlacesToolbarItems:empty{
  height: var(--uc-bm-height);
}

/* Selected tab needs higher z-index now to "hide" the broder below it */
.tabbrowser-tab[selected]{ z-index: 3 !important; }

/* Show when cursor is over main menu popup - this requires :has selector support */
#mainPopupSet:has(> [panelopen]:not(#ask-chat-shortcuts,#tab-preview-panel)) ~ toolbox > .browser-toolbar{
  transition-delay: 100ms !important;
  transform: rotateX(0);
}

/* SELECT TOOLBAR BEHAVIOR */
/* Comment out or delete one of these to disable that behavior */

/* Show when urlbar is focused */
#nav-bar:focus-within + #PersonalToolbar,
#navigator-toolbox > #nav-bar:focus-within{
  transition-delay: 100ms !important;
  transform: rotateX(0);
}

/* Show when cursor is over the toolbar area */
#navigator-toolbox:hover > .browser-toolbar{
  transition-delay: 100ms !important;
  transform: rotateX(0);
}

/* This makes the tab notification box show immediately below tabs, otherwise it would break the layout */
#navigator-toolbox > div{ display: contents }
:where(#titlebar,#navigator-toolbox > #TabsToolbar,#navigator-toolbox > #toolbar-menubar,#tab-notification-deck,.global-notificationbox){
  order: -1;
}
:root[BookmarksToolbarOverlapsBrowser] #navigator-toolbox{
  margin-bottom: calc(-1px - var(--uc-bm-height) + var(--uc-navbar-height)) !important;
  z-index: auto !important;
}
@media (-moz-bool-pref: "browser.fullscreen.autohide"){
  :root[sizemode="fullscreen"] #navigator-toolbox[style*="margin-top"]{
    margin-top: calc(1px - var(--tab-min-height) - 2 * var(--tab-block-margin)) !important;
  }
}

/* Set the following pref to enable compatibility with multi-row_bookmarks.css */
@media (-moz-bool-pref: "userchrome.autohide-bm-and-main-toolbars.multirow-bookmarks-compat.enabled"){
  #navigator-toolbox{ margin-bottom: var(--uc-navbar-height) !important; }
  #PersonalToolbar:not([customizing]){
    min-height: 0 !important;
    margin-bottom: 0;
    height: 0;
    overflow-y: visible !important;
    z-index: 2;
    padding-inline: 0 !important;
  }
  #personal-bookmarks{
    background: inherit;
    height: min-content;
  }
  #PlacesToolbarDropIndicatorHolder{
    pointer-events: none !important;
  }
}