userChrome.css

#contentAreaContextMenu {
 margin: 5px 0 0 5px;
}

#TabsToolbar .tabbrowser-tab[fadein]:not([pinned]) {
  flex-grow: 1 !important;
  max-width: 100% !important;
}

.tab-label {
  -moz-box-flex: 1 !important;
  text-align: center !important;
}

#tabbrowser-tabs, #tabbrowser-tabs > .tabbrowser-arrowscrollbox{ min-height: 0 !important; }
:root:not([customizing]) #tabs-newtab-button,
:root:not([customizing]) #tabbrowser-tabs .tabs-newtab-button{
  transition: 1s;
  -moz-appearance: none !important;
  display: none;
}

#tabbrowser-tabs .tabbrowser-tab[first-visible-tab="true"][last-visible-tab="true"]{
  transition: 1s;
  visibility: collapse;
}

#urlbar input {
  text-align: center;
}