My custom userChrome.css file I use for tweaking Firefox's behaviour

userChrome.css

/*** BEGIN - Override URL bar enlargement ***/

/* Compute new position, width, and padding */

#urlbar[breakout][breakout-extend] {
  top: 5px !important;
  left: 0px !important;
  width: 100% !important;
  padding: 0px !important;
}
/* for alternate Density settings */
[uidensity="compact"] #urlbar[breakout][breakout-extend] {
  top: 3px !important;
}
[uidensity="touch"] #urlbar[breakout][breakout-extend] {
  top: 4px !important;
}

/* Prevent shift of URL bar contents */

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

/* Do not animate */

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

/* Remove shadows */

#urlbar[breakout][breakout-extend] > #urlbar-background {
  box-shadow: none !important;
}

/*** END - Override URL bar enlargement ***/

/*** BEGIN - Autohide bookmarks ***/

#PersonalToolbar{
  --uc-bm-height: 20px; /* Might need to adjust if the toolbar has other buttons */
  --uc-bm-padding: 6px; /* Vertical padding to be applied to bookmarks */
  --uc-autohide-toolbar-delay: 600ms; /* The toolbar is hidden after 0.6s */
}

:root[uidensity="compact"] #PersonalToolbar{ --uc-bm-padding: 1px }
:root[uidensity="touch"] #PersonalToolbar{ --uc-bm-padding: 6px }

#PersonalToolbar:not([customizing]){
  position: relative;
  margin-bottom: calc(0px - var(--uc-bm-height) - 2 * var(--uc-bm-padding));
  transform: rotateX(90deg);
  transform-origin: top;
  transition: transform 135ms linear var(--uc-autohide-toolbar-delay) !important;
  z-index: 1;

}
#PlacesToolbarItems > .bookmark-item{ padding-block: var(--uc-bm-padding) !important; }

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

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

/*** END - Autohide bookmarks ***/