Firefox custom userChrome to make vertical tabs look more like Tab Center.

userChrome.css

#verticaltabs-splitter {
  min-width: 0 !important;
  background: none !important;
}

#verticaltabs-box:hover + #verticaltabs-splitter, #verticaltabs-splitter:hover {
  min-width: 2px !important;
}

#TabsToolbar {
  display:none;
}

.tabbrowser-tab {
  width: 100% !important;
  max-height: 34px !important;
  min-height: 34px !important;
  margin: 0 !important;
  border: 0 !important;
  border-left: 4px solid transparent !important;
  border-radius: 0 !important;
  -moz-appearance: none !important;
  background: transparent !important;
  -moz-box-flex: 0;
  -moz-box-pack: start;
  color: #333 !important;
}

.tabbrowser-tab[selected="true"],
.tabbrowser-tab[multiselect="true"] {
  margin: 0 !important;
  border-left: 4px solid #ff9500 !important;
  border-radius: 0 !important;
  background-color: #dadada !important;
  font-weight: normal;
  font-weight: 600 !important;
  text-shadow: none;
}

.tabbrowser-tab[selected="true"]:-moz-window-inactive,
.tabbrowser-tab[multiselect="true"]:-moz-window-inactive {
  background-color: InactiveCaption !important;
  color: InactiveCaptionText !important;

  /* gnome/gtk/cleartype doesn't seem to have inactive states for selected items, lets try this */
}

.tabbrowser-tab[pinned] {
  border-left: 4px solid #0996f8 !important;
}

.tabbrowser-tab:not([selected="true"]):hover {
  margin: 0 !important;
  background-color: #ebebeb !important;
}

.tabbrowser-tab[pinned] .tab-close-button,
.tabbrowser-tab:not(:hover) .tab-close-button {
  visibility: collapse !important;
}

.tab-content {
  min-height: 34px;
  max-height: 34px;
  margin: 0 !important;
  padding: 0 !important;
}


.tabbrowser-tab[busy] .tab-throbber {
  display: -moz-box !important;
}


.tabbrowser-tab[pinned] .tab-close-button,
.tabbrowser-tab:not(:hover) .tab-close-button {
  visibility: collapse !important;
}

.tab-close-button {
  display: -moz-box !important;
  width: 34px;
  height: 34px;
  margin: 0 0 0 3px !important;
  color:#FFFFFF !important;
}

.tab-close-button:hover {
  background-color: #d94141 !important;
}

.tab-close-button:hover::before {
  content: "×";
  display: block;
  font-size: 20px;
  font-weight: bold;
  margin-top: -2px;
}

.tab-close-button > .toolbarbutton-icon {
  display: none;
}