Get the multi-row tab bar back in Firefox

github.com: MrOtherGuy/firefox-csshacks
github.com: Aris-t2/CustomCSSforFx

C:\Users\$user\AppData\Roaming\Comodo\IceDragon\Profiles\ $profile_name .default\chrome\userChrome.css

C:\Users\$user\AppData\Roaming\Mozilla\Firefox\Profiles\$profile_name.default\chrome\ userChrome.css

Sample userChrome.css for Firefox on Linux:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* Global UI font */
* {
    font-family: Noto Sans !important;
    font-style: SemiCondensed Light !important;
    font-size: 11pt !important;
}


/* Developer tools code font */
.CodeMirror {
    font-family: Ubuntu Mono, monospace !important;
    font-size: 12pt !important;
}


/* Bookmark font */
#personal-bookmarks .bookmark-item menupopup * {
    font-family: Noto Sans !important;
    font-style: SemiCondensed Light !important;
    font-size:14pt !important;
}



/* Remove URL bar zoom button */
#urlbar-zoom-button {
    display:none!important
}


/* Remove tab blur */
.tab-label-container[textoverflow]:not([pinned]) {
  mask-image: unset !important;
}


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


/* Active Tab Background Color */
.tab-background[selected="true"] {
        background-attachment: none!important;
        background-color: rgb(91, 255, 107)!important;
        background-image: none!important;
        background-position: none!important;
        background-repeat: none!important;
        background-size: none!important;
}


/* Multi-row tabs
https://github.com/Aris-t2/CustomCSSforFx/blob/master/classic/css/tabs/tabs_multiple_lines_v2.css
*/
:root{
  --tabs-lines: 4;
  --tab_min_width_mlt: 80px;
  --tab_max_width_mlt: 200px;
  /* --tab-min-height_mlt: var(--tab-min-height,32px); */ /* set own value here, if used without configuration files */
 --tab-min-height_mlt: 22px;
}

#tabbrowser-tabs{
  min-height: unset !important;
  padding-inline-start: 0px !important
}

/* Test for Firefox > 66 */
@supports (inset-block:auto){
  #tabbrowser-tabs > .tabbrowser-arrowscrollbox > .arrowscrollbox-scrollbox{
    display: flex;
    flex-wrap: wrap;
    overflow-y: auto;
    max-height: calc(var(--tab-min-height_mlt) * var(--tabs-lines));
  }
  #tabbrowser-tabs > .tabbrowser-arrowscrollbox {
    overflow: -moz-hidden-unscrollable;
    display: block;
        margin-bottom:-1px !important;
  }
}

/* Test for Firefox < 66 */
@supports not (inset-block:auto){
  #tabbrowser-tabs > .tabbrowser-arrowscrollbox{
    min-height: unset !important;
  }
  #tabbrowser-tabs .scrollbox-innerbox{
    display: flex;
    flex-wrap: wrap;
  }
  #tabbrowser-tabs .arrowscrollbox-scrollbox {
    overflow: -moz-hidden-unscrollable;
    display: block;
  }
}

.tabbrowser-tab{
  height: var(--tab-min-height_mlt);
}

#tabbrowser-tabs .tabbrowser-tab[pinned]{
  position: static !important;
  margin-inline-start: 0px !important;
}

.tabbrowser-tab[fadein]:not([pinned]) {
  flex-grow: 1;
  min-width: var(--tab_min_width_mlt) !important;
  max-width: var(--tab_max_width_mlt) !important;
}

.tabbrowser-tab > stack{
  width: 100%;
  height: 100%;
}

#tabbrowser-tabs .scrollbutton-up,
#tabbrowser-tabs .scrollbutton-down,
#alltabs-button,
:root:not([customizing]) #TabsToolbar #new-tab-button,
#tabbrowser-tabs spacer,
.tabbrowser-tab::after{
  display: none !important;
}

#tabbrowser-tabs[overflow="true"] > .tabbrowser-arrowscrollbox > .tabs-newtab-button,
#tabbrowser-tabs:not([hasadjacentnewtabbutton]) > .tabbrowser-arrowscrollbox > .tabs-newtab-button,
#TabsToolbar[customizing="true"] #tabbrowser-tabs > .tabbrowser-arrowscrollbox > .tabs-newtab-button {
  visibility: visible !important;
}

/* hide private window indicator, window controls and titlebar placeholders */ #main-window[tabsintitlebar] #TabsToolbar .private-browsing-indicator,
#main-window[tabsintitlebar] #TabsToolbar #window-controls,
#main-window[tabsintitlebar] #TabsToolbar .titlebar-spacer[type="pre-tabs"],
#main-window[tabsintitlebar] #TabsToolbar .titlebar-spacer[type="post-tabs"] {
  display: none !important;
}
#TabsToolbar .titlebar-placeholder[type="pre-tabs"],
#TabsToolbar .titlebar-placeholder[type="post-tabs"] {
  opacity: 0 !important;
}

/* hide tab borders set by Firefox 58+ to solve 'blank space below tabs' issue */
.tabbrowser-tab::after,
.tabbrowser-tab::before {
  border-left: unset !important;
  border-image: unset !important;
  border-image-slice: unset !important;
  border: 0 !important;
}

/* Fx66+ tab close fix */
#TabsToolbar #tabbrowser-tabs .tabbrowser-tab:not([pinned]) .tab-close-button {
  visibility: hidden !important;
  display: block !important;
}
#TabsToolbar #tabbrowser-tabs .tabbrowser-tab[pinned] .tab-close-button {
  visibility: hidden !important;
  display: block !important;
  -moz-margin-start: -18px !important;
}

/* fix scrollbar */
#main-window[tabsintitlebar] #tabbrowser-tabs {
  -moz-window-dragging: no-drag !important;
}

/* End Multi-row Tabs */

sample userChrome.css for Firefox on Windows:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* Bookmark font */
#personal-bookmarks .bookmark-item menupopup * {
font-size:14pt !important;
}

/* Multi-row tabs */
/* Tab reordering will not work and can't be made to work */
/* You can use multi-row_tabs_window_control_patch.css to move window controls to nav-bar*/

/* It's recommended to move tabs new-tab-button outside tabs toolbar */

/* Change the --multirow-n-rows to change maximum number of rows before the rows will start to scroll  */
/* Scrollbar can't be clicked but the rows can be scrolled with mouse */
/* This maximum visible rows won't work before Fx66 */
/* So this setting does nothing on Fx65 and all tab rows will be shown */
:root{
    --multirow-n-rows: 3;
    --multirow-tab-min-width: 100px;
    --multirow-tab-dynamic-width: 1; /* Change to 0 for fixed-width tabs using the above width. */
}

#tabbrowser-tabs{
  min-height: unset !important;
  padding-inline-start: 0px !important
}

/* Test for Firefox > 66 */
@supports (inset-block:auto){
  #tabbrowser-tabs > .tabbrowser-arrowscrollbox > .arrowscrollbox-scrollbox{
    display: flex;
    flex-wrap: wrap;
    overflow-y: auto;
    max-height: calc(var(--tab-min-height) * var(--multirow-n-rows));
    scrollbar-color: var(--toolbar-bgcolor) var(--lwt-accent-color);
    scrollbar-width: thin;
  }
  #tabbrowser-tabs > .tabbrowser-arrowscrollbox {
    overflow: -moz-hidden-unscrollable;
    display: block;
  }
}

/* Test for Firefox < 66 */
@supports not (inset-block:auto){
  #tabbrowser-tabs > .tabbrowser-arrowscrollbox{
    min-height: unset !important;
  }
  #tabbrowser-tabs .scrollbox-innerbox{
    display: flex;
    flex-wrap: wrap;
  }
  #tabbrowser-tabs .arrowscrollbox-scrollbox {
    overflow: -moz-hidden-unscrollable;
    display: block;
  }
}

.tabbrowser-tab{ height: var(--tab-min-height); }
#tabbrowser-tabs .tabbrowser-tab[pinned]{
  position: static !important;
  margin-inline-start: 0px !important;
}

.tabbrowser-tab[fadein]:not([pinned]){
  min-width: var(--multirow-tab-min-width) !important;
  flex-grow: var(--multirow-tab-dynamic-width);
  /*
  Uncomment to enable full-width tabs, also makes tab dragging a tiny bit more sensible
  Don't set to none or you'll see errors in console when closing tabs
  */
  /*max-width: 100vw !important;*/
}

.tabbrowser-tab > stack{ width: 100%; height: 100% }

#tabbrowser-tabs .scrollbutton-up,
#tabbrowser-tabs .scrollbutton-down,
#alltabs-button,
:root:not([customizing]) #TabsToolbar #new-tab-button,
#tabbrowser-tabs spacer,
.tabbrowser-tab::after{ display: none !important }

/* End multi-row tabs */


/* Hide close button on tabs */
#TabsToolbar #tabbrowser-tabs .tabbrowser-tab:not([pinned]) .tab-close-button {
  visibility: hidden !important;
  display: block !important;
  opacity: 0 !important;
  -moz-margin-end: -16px !important;
}

#TabsToolbar #tabbrowser-tabs .tabbrowser-tab[pinned] .tab-close-button {
  visibility: hidden !important;
  display: block !important;
  opacity: 0 !important;
  -moz-margin-start: -18px !important;
}
/* End hide close button on tabs */
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