My userChrome.css after updating to Firefox 57

And another update for Firefox 60. Class .tabbrowser-tabs was replaced by id #tabbrowser-tabs. Thanks to this reddid-thread.
When updating Firefox to version 57 some plugins stopped working. Most importantly I am missing Tab Mix Plus now. That's why I spent some time getting at least the multirow feature back - though still missing scrolling on tabbar.

I found some lines on reddit and Github (check this) and modified them a bit for my needs - multirow, toolbar order, layout... Here's what I got:

firefox_multirow

And finally my userChrome.css looks like this:

/* Tabs toolbar adjustment */
#main-window[tabsintitlebar] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar, 
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#TabsToolbar{
  -moz-padding-start: 2px !important;
}
#main-window[tabsintitlebar][sizemode="normal"]:not([inFullscreen]) #TabsToolbar {
  margin-top: 0px !important;
}

/* Fix for application/ hamburger button in titlebar */
#main-window[tabsintitlebar][inFullscreen] #toolbar-menubar[autohide="true"][inactive="true"] ~ #nav-bar  #PanelUI-button {
  visibility: collapse !important;
}

/* Tab height */
#tabbrowser-tabs,
#tabbrowser-tabs > .tabbrowser-arrowscrollbox,
#tabbrowser-tabs[positionpinnedtabs] > .tabbrowser-tab[pinned] {
  min-height: 25px !important;
}

/* Multi-row */
.tabbrowser-tab {
    min-width:118px !important;
}
.tabbrowser-tab,.tab-background {
    height:29px;
}
.tab-stack {
    width: 100%;
}
#tabbrowser-tabs .scrollbox-innerbox {
    display: flex;
    flex-wrap: wrap;
}
#tabbrowser-tabs .arrowscrollbox-scrollbox {
    overflow: visible;
    display: block;
}
/* Multi-row (end) */

/* Toolbar order (start) ************************************/
#print-preview-toolbar,
#printedit-toolbar,
#titlebar {
  -moz-box-ordinal-group: 0 !important;
}
#navigator-toolbox #toolbar-menubar {
  -moz-box-ordinal-group: 1 !important;
}
/* navigation toolbar */
#navigator-toolbox #nav-bar {
  -moz-box-ordinal-group: 2 !important;
}
/* bookmarks toolbar */
#navigator-toolbox #PersonalToolbar {
  -moz-box-ordinal-group: 3 !important;
}
/* 3rd party toolbars */
#navigator-toolbox toolbar {
  -moz-box-ordinal-group: 10 !important;
}
/* tabs toolbar */
#navigator-toolbox #TabsToolbar {
  -moz-box-ordinal-group: 100 !important;
}
/* toolbar order (end) **************************************/


/* Toolbar borders */
#main-window #navigator-toolbox::after {
  border-bottom: unset !important;
  opacity: 0 !important;
}
#TabsToolbar{
  margin-bottom: 0px !important;
  border-top: 1px solid #5f7181 !important;
  border-bottom: 1px solid #5f7181 !important;
}
#main-window:not([inFullscreen="true"])[tabsintitlebar] #TabsToolbar{
  -moz-margin-end: 0px !important;
}
#main-window[tabsintitlebar] #TabsToolbar .titlebar-placeholder{
  visibility: collapse !important;
}
#main-window[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"][inactive="true"],
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] {
  margin-top: 21px !important;
}
#tabbrowser-tabs .tab-drop-indicator {
  margin-bottom: 0px !important;
}
/* Toolbar borders (end) */

/* Tab border */
#TabsToolbar:not(:-moz-lwtheme) .tabs-newtab-button,
#TabsToolbar:not(:-moz-lwtheme) .tabbrowser-tab .tab-content {
  border-bottom: 1px solid #919ba4 !important;
  border-right: 1px solid #919ba4 !important;
}

/* Hide close button */
.tab-close-button {
  visibility: collapse !important;
}

/* Move the tab line to bottom of a tab */
.tabbrowser-tab .tab-line,
#TabsToolbar,
.tabbrowser-tab .tab-bottom-line {
	 -moz-box-ordinal-group: 2;
}