Firefox 58+ Multiple line tabs

userChrome.css

/* Firefox type in urlbar about:support */
/* and click in Open Folder at Profile Folder line */
/* Create folder chrome and copy this file there */
/**
All credits for this file go to https://www.reddit.com/r/firefox/comments/726p8u/multirow_tabs_firefox_ignores_mozboxflex/dngb8qf/
*/

.tabbrowser-tab:not([pinned]) {
    flex-grow:1;
    min-width:150px;
}
.tabbrowser-tab,.tab-background {
    height:var(--tab-min-height);
}
.tab-stack {
    width: 100%;
}
#tabbrowser-tabs .scrollbox-innerbox {
    display: flex;
    flex-wrap: wrap;
}
#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;
}