Firefox custom userChrome - fullscreen tabs only, hover over shows address bar, & bookmarks bar.

userChrome.css

/* ────────────────────────────────────────────────
   AUTOHIDE URL BAR + BOOKMARKS BAR (FULLSCREEN ONLY)
   ──────────────────────────────────────────────── */

/* Default fullscreen cleanup: hide everything but tabs */
:root[inFullscreen="true"] #navigator-toolbox > *:not(#TabsToolbar) {
  max-height: 0 !important;
  min-height: 0 !important;
  opacity: 0 !important;
  overflow: hidden !important;
  transition: all 0.25s ease-in-out !important;
}

/* Fully collapse the URL bar components */
:root[inFullscreen="true"] #nav-bar,
:root[inFullscreen="true"] #urlbar-container,
:root[inFullscreen="true"] #urlbar,
:root[inFullscreen="true"] #urlbar-background {
  max-height: 0 !important;
  min-height: 0 !important;
  opacity: 0 !important;
  overflow: hidden !important;
  transition: all 0.25s ease-in-out !important;
}

/* Reveal toolbars + URL bar when hovering over TabsToolbar or toolbox area */
:root[inFullscreen="true"] #TabsToolbar:hover ~ *,
:root[inFullscreen="true"] #navigator-toolbox:hover > *:not(#TabsToolbar),
:root[inFullscreen="true"] #navigator-toolbox:hover #urlbar-container,
:root[inFullscreen="true"] #navigator-toolbox:hover #urlbar,
:root[inFullscreen="true"] #navigator-toolbox:hover #urlbar-background {
  max-height: 100px !important;
  min-height: 24px !important;
  opacity: 1 !important;
  transition-delay: 0s !important;
}

/* Keep the TabsToolbar always visible, compact, and static */
:root[inFullscreen="true"] #TabsToolbar {
  visibility: visible !important;
  min-height: 28px !important;
  max-height: 28px !important;
  transition: none !important;
}

/* Optional: smooth fade-in/out and shadow separation */
:root[inFullscreen="true"] #navigator-toolbox {
  box-shadow: 0 2px 5px rgba(0,0,0,0.2) !important;
}