Smaller and tighter firefox tabs, similar to how chrome/edge renders them.

userChrome.css

.tab-background {
  border-radius: 10px 10px 0 0 !important;
  margin-bottom: 0px !important;
}

.tabbrowser-tab:not([selected=true]):not([multiselected=true]) .tab-background {
  background-color: color-mix(in srgb, currentColor 5%, transparent);
}

.tabbrowser-tab {
  padding: 0px !important;
  max-height: 5px !important;
}

menupopup>menu,
menupopup>menuitem {
  padding-block: 2px !important;
}

:root {
  --arrowpanel-menuitem-padding: 2px !important;
}