If you want to arrange Firefox tabs in multiple rows.
add the following code to your userChrome.css.


.tabbrowser-tabs > hbox {
  display: block !important;
}

.tabbrowser-tabs tab {
  -moz-appearance: none !important;
  display: inline !important;
  width: 16% !important;
  min-width: 16% !important;
  max-width: 16% !important;
}