Multi-Row Tabs in Firefox 57 und später

Worum geht es?

Was ist zu tun?

1. Im Profil-Verzeichnis C:\Users\user\AppData\Roaming\Mozilla\Firefox\Profiles\PROFILNAME.default\ das Verezichnis chrome erstellen
2. In diesem Verzeichnis das File userChrome.css erstellen
3. Dort folgendes reinschreiben und Firefox neu starten

/*
The class .tabbrowser-tabs has been replaced with id #tabbrowser-tabs
changed selectors accordingly
*/
.tabbrowser-tab:not([pinned]) {
    flex-grow:1;
    min-width:94px !important; /* Needed important tag, width can be whatever you like */
    max-width: 94px !important; /* none - Makes the tabs always fill the toolbar width */
}
.tabbrowser-tab,.tab-background {
    height:var(--tab-min-height);
}
.tab-stack {
    width: 100%;
}
#tabbrowser-tabs .scrollbox-innerbox {
    display: flex;
    flex-wrap: wrap;
    overflow-y: auto !important;
    min-height: var(--tab-min-height);
    max-height: calc(3*var(--tab-min-height)) !important; /* 3 rows */
}
#tabbrowser-tabs .arrowscrollbox-scrollbox {
    overflow: visible; 
    display: block;
}

#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;
}
#main-window[sizemode="maximized"] #TabsToolbar{
    margin-left:var(--tab-min-height);
}
#titlebar:active{
    margin-bottom:0 !important;
}
#titlebar:active #titlebar-content{
    margin-bottom:var(--tab-min-height) !important;
}
#tabbrowser-tabs .scrollbutton-up,#tabbrowser-tabs .scrollbutton-down,#alltabs-button,.tabbrowser-tab:not([fadein]){
    display: none;
}

/* This enables maximum width before scrollbar is shown */
#main-window[tabsintitlebar] #tabbrowser-tabs {
    -moz-window-dragging: no-drag;
}

/* Show close button on hover */
.tab-close-button{ display: none }
.tabbrowser-tab:not([pinned]):hover .tab-close-button{
    display: -moz-box !important
}

/* New activity in tab */
.tabbrowser-tab[titlechanged] .tab-text.tab-label {
    color: #990000 !important;
    font-size: 10px !important;
    font-style: italic !important;
}

/* Selected (loaded) */
.tabbrowser-tab[selected="true"] .tab-label {
    color: #eee !important;
    font-size: 12px !important;
    font-style: normal !important;
    font-weight: bold !important;
    text-shadow: 1px 2px 1px #000 !important;
}

/* Loaded but unread (not selected) */
.tabbrowser-tab:not([selected="true"])[unread] .tab-label {
    color: #2565BA !important;
    font-size: 10px !important;
    font-style: normal !important;
}

/* Not loaded yet (also visited) */
.tabbrowser-tab:not([selected="true"])[pending] .tab-label,
.tabbrowser-tab:not([selected="true"])[visited] .tab-label {
    color: #8c8c8c !important;
    font-size: 10px !important;
    font-style: italic !important;
}

/* loading */
.tabbrowser-tab[busy] .tab-text.tab-label {
    color: #006bb3 !important;
    font-size: 10px !important;
    font-style: italic !important;
}

Quellen

• https://www.reddit.com/r/FirefoxCSS/comments/7dclp7/multirow_tabs_in_ff57/dpwsh6s/