Multi-row tabs on Firefox https://www2.kokoro-navi.com/diary4/2019/03/firefox-66-tab-mix-plus/

userChrome.css

:root {
  --multirow-n-rows: 3;
}
#tabbrowser-tabs {
  min-height: unset !important;
  padding-inline-start: 0px !important;
}
#tabbrowser-tabs > .tabbrowser-arrowscrollbox > .arrowscrollbox-scrollbox {
  display: flex;
  flex-wrap: wrap;
  overflow-y: auto;
  max-height: calc(var(--tab-min-height) * var(--multirow-n-rows));
  scrollbar-color: var(--toolbar-bgcolor) var(--lwt-accent-color);
  scrollbar-width: thin;
}
#tabbrowser-tabs > .tabbrowser-arrowscrollbox {
  overflow: -moz-hidden-unscrollable;
  display: block;
}
.tabbrowser-tab {
  height: var(--tab-min-height);
}
#tabbrowser-tabs .tabbrowser-tab[pinned] {
  position: static !important;
  margin-inline-start: 0px !important;
}
.tabbrowser-tab > stack {
  width: 100%;
  height: 100%;
}
#tabbrowser-tabs .scrollbutton-up,
#tabbrowser-tabs .scrollbutton-down,
#alltabs-button,
:root:not([customizing]) #TabsToolbar #new-tab-button,
#tabbrowser-tabs spacer,
.tabbrowser-tab::after {
  display: none !important;
}
.tabbrowser-tab[fadein]:not([pinned]) {
  min-width: 100px !important;
  flex-grow: 1;
}

/* don't show tab close buttons */
.tabbrowser-tab:not([selected="true"]) .tab-close-button {
  display: none !important;
}