Make the Firefox 89 “compact” mode actually compact

readme.md

userChrome.css

:root[uidensity="compact"] {
  --tab-border-radius: 5px 5px 0 0 !important;
  --tab-button-border-radius: 5px;
  --tab-min-height: 24px;
  --toolbar-start-end-padding: 5px !important;
  --toolbarbutton-inner-padding: 5px !important;
  --toolbarbutton-outer-padding: 1px !important;
}

:root[uidensity="compact"] .tab-background {
  margin-bottom: 0 !important;
  margin-top: 0 !important;
  overflow: hidden;
}

:root[uidensity="compact"] .tab-close-button {
  border-radius: var(--tab-button-border-radius) !important;
}

:root[uidensity="compact"] .tab-context-line {
  border-radius: 0 !important;
  margin: 0 !important;
}

:root[uidensity="compact"] .tabbrowser-tab {
  padding-inline: 0 1px !important;
}

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

:root[uidensity="compact"] .titlebar-spacer[type="pre-tabs"] {
  display: none;
}

:root[uidensity="compact"]
  #tabbrowser-tabs[haspinnedtabs]:not([positionpinnedtabs])
  > #tabbrowser-arrowscrollbox
  > .tabbrowser-tab[first-visible-unpinned-tab] {
  margin-inline-start: 0px !important;
}