userChrome.css

/* remove radius from buttons and tabs */
*|*:root {
  --toolbarbutton-border-radius: 0 !important;
  --tab-border-radius: 0 !important;
  --toolbarbutton-outer-padding: 0 !important;
  --toolbarbutton-inner-padding: 8px !important;
  --toolbar-start-end-padding: 0 !important;
}

/* remove margin fron tabs */
.tab-background {
  margin-block: 0 !important;
}

/* remove padding between tabs */
.tabbrowser-tab {
  padding-inline: 0 !important;
}

/*fix for when titlebar gets taller when there are many tabs*/
#tabbrowser-arrowscrollbox {
  height: var(--tab-min-height);
}

/* remove titlebar top padding */
#titlebar {
  margin-top: 0 !important;
}

/* remove gaps in tab bar start and end */
.titlebar-spacer[type="pre-tabs"] {
  width: 0 !important;
}

.titlebar-spacer[type="post-tabs"] {
  width: 0 !important;
}

/* This reorders toolbar to place tabs below other toolbars. Requires Firefox 133+ */
@media (-moz-bool-pref: "userchrome.force-window-controls-on-left.enabled"),
  -moz-pref("userchrome.force-window-controls-on-left.enabled") {
  #nav-bar > .titlebar-buttonbox-container {
    order: -1 !important;
  }
}
> .titlebar-buttonbox {
  flex-direction: row-reverse;
}

@media not (-moz-bool-pref: "sidebar.verticalTabs"),
  not -moz-pref("sidebar.verticalTabs") {
  .global-notificationbox,
  #tab-notification-deck,
  #TabsToolbar {
    order: 1;
  }

  #TabsToolbar > :is(.titlebar-spacer, .titlebar-buttonbox-container) {
    display: none;
  }

  :root[sizemode="fullscreen"] #nav-bar > .titlebar-buttonbox-container {
    display: flex !important;
  }

  :root:is([tabsintitlebar], [customtitlebar])
    #toolbar-menubar:not([autohide="false"])
    ~ #nav-bar {
    > .titlebar-buttonbox-container {
      display: flex !important;
    }

    :root[sizemode="normal"] & {
      > .titlebar-spacer {
        display: flex !important;
      }
    }

    :root[sizemode="maximized"] & {
      > .titlebar-spacer[type="post-tabs"] {
        display: flex !important;
      }

      @media (-moz-bool-pref: "userchrome.force-window-controls-on-left.enabled"),
        -moz-pref("userchrome.force-window-controls-on-left.enabled"),
        (-moz-gtk-csd-reversed-placement),
        (-moz-platform: macos) {
        > .titlebar-spacer[type="post-tabs"] {
          display: none !important;
        }

        > .titlebar-spacer[type="pre-tabs"] {
          display: flex !important;
        }
      }
    }
  }
}

/* urlbar lower */
#urlbar[focused] {
  border: 0px solid rgb(30, 144, 255) !important;
}

/* height urlbar-container */
#urlbar-container {
  padding-block: 0px !important;
  margin-inline: 0px !important;
}

/* folders color */
.bookmark-item[container],
treechildren::-moz-tree-image(container) {
  fill: DarkSeaGreen !important;
}

.bookmark-item[container="true"] [label="a"] {
  fill: blue !important;
}

.bookmark-item[container="true"] [label="b"] {
  fill: DarkSeaGreen !important;
}

.bookmark-item[container="true"] [label="c"] {
  fill: chocolate !important;
}

.bookmark-item[container="true"] [label="d"] {
  fill: olive !important;
}

.bookmark-item[container="true"] [label="e"] {
  fill: darkgreen !important;
}

.bookmark-item[container="true"] [label="f"] {
  fill: darkslategray !important;
}

.bookmark-item[container="true"] [label="g"] {
  fill: slategray !important;
}

.bookmark-item[container="true"] [label="h"] {
  fill: saddlebrown !important;
}

.bookmark-item[container="true"] [label="i"] {
  fill: teal !important;
}

.bookmark-item[container="true"] [label="j"] {
  fill: cadetblue !important;
}

.bookmark-item[container="true"] [label="k"] {
  fill: darkcyan !important;
}

.bookmark-item[container="true"] [label="l"] {
  fill: red !important;
}

/* hide close on tab */
.tabbrowser-tab:not([pinned]):not(:hover) .tab-close-button {
  visibility: collapse !important;
}

.tabbrowser-tab:not([pinned]):hover .tab-close-button {
  visibility: visible !important;
  display: block !important;
}

/* hide urlbar border */
#urlbar {
  border-color: transparent !important;
  border-width: 0px !important;
}

/* hide folder-text on bookmarks toolbar */
#personal-bookmarks .bookmark-item > .toolbarbutton-text {
  display: none !important;
}

/* Hide star button (bookmark) */
#star-button-box {
  display: none !important;
}

/* Active tab color */
.tab-background[selected] {
  background-color: #525151 !important;
}

.titlebar-button:not(#toolbar-menubar .titlebar-button) {
  display: none !important;
}