UserChrome.css

#tabbrowser-tabs .tabbrowser-arrowscrollbox,
#tabbrowser-tabs .arrowscrollbox-scrollbox {
    display: block;
}

.scrollbutton-up,
.arrowscrollbox-overflow-start-indicator,
.scrollbutton-down,
.arrowscrollbox-overflow-end-indicator {
    display: none !important;
}

#tabbrowser-tabs .arrowscrollbox-scrollbox .scrollbox-innerbox {
    display: flex;
    flex-wrap: wrap;
    /*
    display: block;
    */
    overflow-y: auto !important;
    min-height: var(--tab-min-height); /* default */
    max-height: calc(5*var(--tab-min-height)) !important;
}
#tabbrowser-tabs .tabbrowser-tab:not([pinned]) {
    flex-grow: 1;
    flex-wrap:wrap;
    min-width: 150px;
    vertical-align: bottom !important;
}
#tabbrowser-tabs .tabbrowser-tab,
#tabbrowser-tabs .tabbrowser-tab .tab-stack .tab-background {
    height: var(--tab-min-height);
}

#tabbrowser-tabs .tabbrowser-tab .tab-stack {
    width: 100%;
}

#titlebar-buttonbox {
    display: block !important;
    vertical-align: top !important;
}

#main-window[tabsintitlebar] #tabbrowser-tabs {
    -moz-window-dragging: no-drag;   
}
/* Show Tab Close buttons only when hovered */
#tabbrowser-tabs > .tabbrowser-tab:not([pinned="true"]) > .tab-stack > .tab-content > .tab-close-button {
  visibility: collapse !important;
  opacity: 0 !important;
  transition: all 250ms ease-in-out !important;
}
#tabbrowser-tabs > .tabbrowser-tab:not([pinned="true"]):hover > .tab-stack > .tab-content > .tab-close-button {
  visibility: visible !important;
  opacity: 1 !important;
  transition: all 250ms ease-in-out !important;;
}
#tabbrowser-tabs > .tabbrowser-tab:not([pinned="true"]) > .tab-stack > .tab-content > .tab-close-button {
  display: -moz-box !important;
}