#main-window {
  /* Remove animation when changing window from active <-> inactive */
  transition: none !important;
}

.titlebar-placeholder[type="pre-tabs"] {
  /**
   * Reduce left padding between traffic-light and tabs, so that left and right
   * traffic light padding is equal, and there is more space for tabs.
   */
  width: 12px !important;
}

.tabbrowser-tab:-moz-any([image], [pinned]) > .tab-stack > .tab-content[attention]:not([selected="true"]),
.tabbrowser-tab > .tab-stack > .tab-content[pinned][titlechanged]:not([selected="true"]) {
  /* Disable tab indicator */
  background-image: none !important;
}

#TabsToolbar {
  /* Restore native macOS styling for titlebar */
  -moz-appearance: -moz-window-titlebar !important;
  background-color: inherit !important;
  color: rgb(47, 47, 47) !important;
}

#TabsToolbar:-moz-window-inactive,
#TabsToolbar:-moz-window-inactive .tabbrowser-tab[visuallyselected="true"] {
  /* Restore native macOS styling for titlebar */
  color: rgb(176, 176, 176) !important;
}

#tabbrowser-tabs {
  /* Use native macOS highlight color for active tab highlight line */
  --tab-line-color: Highlight !important;
}

.tab-label {
  /* Increase tab title font size */
  font-size: 1.25em !important;
}

#main-window {
  /* Remove animation when changing window from active <-> inactive */
  transition: none !important;
}

.titlebar-placeholder[type="pre-tabs"] {
  /**
   * Reduce left padding between traffic-light and tabs, so that left and right
   * traffic light padding is equal, and there is more space for tabs.
   */
  width: 12px !important;
}

.tabbrowser-tab:-moz-any([image], [pinned]) > .tab-stack > .tab-content[attention]:not([selected="true"]),
.tabbrowser-tab > .tab-stack > .tab-content[pinned][titlechanged]:not([selected="true"]) {
  /* Disable tab indicator */
  background-image: none !important;
}

#TabsToolbar {
  /* Restore native macOS styling for titlebar */
  -moz-appearance: -moz-window-titlebar !important;
  background-color: inherit !important;
  color: rgb(47, 47, 47) !important;
}

#TabsToolbar:-moz-window-inactive,
#TabsToolbar:-moz-window-inactive .tabbrowser-tab[visuallyselected="true"] {
  /* Restore native macOS styling for titlebar */
  color: rgb(176, 176, 176) !important;
}

#tabbrowser-tabs {
  --tab-line-color: transparent !important;
}

.tab-label {
  /* Increase tab title font size */
  font-size: 1.25em !important;
}

Hide active tab top highlight line
firefox: Hide active tab top highlight line
