Firefox CSS + Pill Shaped Tabs & More

/******************** 0. Font Sizes ********************/
/* Scoped font size */
:root {
  font-size: 14px !important;
  --tab-min-height: 28px !important;
}

/* Address bar font size */
#nav-bar {
  font-size: 12px !important;
}

/* Bookmarks (All Items - folders) */
.bookmark-item[container] label {
  font-size: 14.5px !important;
}


/******************** 1. Toolbar Order ********************/
#navigator-toolbox {
  display: flex;
  flex-direction: column;
}

#nav-bar { order: 1; }
#PersonalToolbar { order: 2; }
#TabsToolbar { order: 3; }


/******************** 2. Tab Bar Changes ********************/

.tabbrowser-tab {
  padding-block: 0 !important;
}

/* Pill-shaped tabs */
.tabbrowser-tab .tab-background {
  border-radius: 999px !important;
  margin-block: 2px !important;
  padding-inline: 8px !important;
}

/* Active tab */
.tabbrowser-tab[selected="true"] .tab-background {
  background-color: var(
    --tab-selected-bgcolor,
    var(--toolbar-bgcolor)
  ) !important;
  border: 1px solid var(--toolbar-field-border-color, #aaa) !important;
}

/* Active tab font weight */
.tabbrowser-tab[selected="true"] .tab-label {
  font-weight: 500 !important;
}

/* Inactive tabs */
.tabbrowser-tab:not([selected="true"]) .tab-background {
  background-color: var(
    --tab-background-color,
    rgba(150, 150, 150, 0.15)
  ) !important;
  border: 1px solid transparent !important;
}

/* Close button */
.tabbrowser-tab .tab-close-button {
  width: 14px !important;
  height: 14px !important;
  padding: 0 !important;
  margin-block: 2px !important;
  margin-inline-start: 2px !important;
  margin-inline-end: 1px !important;
}

/* Close button hover shape */
.tabbrowser-tab .tab-close-button:hover {
  border-radius: 50% !important;
}


/******************** 3. Bookmarks Changes ********************/

/* Remove folder icon from bookmark folders */
#PersonalToolbar toolbarbutton.bookmark-item[container] .toolbarbutton-icon {
  display: none !important;
}

/* Extra padding for bookmark folder labels */
#PersonalToolbar toolbarbutton.bookmark-item[container] .toolbarbutton-text {
  padding-left: 4px !important;
  padding-right: 4px !important;
}

/* Add space below bookmarks toolbar */
#PersonalToolbar {
  padding-bottom: 3px !important;
}

/* Center bookmarks */
#PersonalToolbar #PlacesToolbarItems {
  display: flex !important;
  justify-content: center !important;
  width: 100% !important;
}


/******************** 4. Navigation Buttons ********************/

/* Hide back & forward buttons */
#back-button,
#forward-button {
  display: none !important;
}