userChrome.css

/* MULTI-ROW TABS
   based on https://www.reddit.com/r/FirefoxCSS/comments/7dclp7/multirow_tabs_in_ff57/
   If tabs are too narrow, go into about:config and adjust browser.tabs.tabMinWidth
*/

/* General Rules */
#tabbrowser-tabs .arrowscrollbox-scrollbox {
  overflow: visible;
  display: block;
}
#tabbrowser-tabs .scrollbox-innerbox {
  display: flex;
  flex-wrap: wrap;
  min-height: var(--tab-min-height);
  max-height: calc((5 * var(--tab-min-height)) + 1px) !important; /* up to 5 rows */
}
.tabbrowser-tab, .tab-background {
  height: var(--tab-min-height);
}
.tab-stack {
  width: 100%;
}
#main-window[sizemode="normal"] #TabsToolbar {
  border-left: 1px solid #888 !important;
}
/* Title bar rules */
#titlebar, #titlebar-buttonbox {
  height: var(--tab-min-height) !important;
}
#titlebar {
  margin-bottom: calc(var(--tab-min-height)*-1) !important;
}
#main-window[sizemode="maximized"] #titlebar {
  margin-bottom: calc(6px + var(--tab-min-height)*-1) !important;
}
#titlebar:active {
  margin-bottom: 0 !important;
}
#titlebar:active #titlebar-content {
  margin-bottom: var(--tab-min-height) !important;
}
/* Remove unnecessary buttons and the blank areas at left and right */
#tabbrowser-tabs .scrollbutton-up, #tabbrowser-tabs .scrollbutton-down,
#alltabs-button, .tabbrowser-tab:not([fadein]),
.titlebar-placeholder[type="pre-tabs"],
.titlebar-placeholder[type="post-tabs"] {
  display: none;
}
/* Ensure a border between tabs (#aaa is a light gray) */
.tabbrowser-tab:not(:last-of-type) {
  border-right: 1px solid #aaa !important;
}