How to add multi-tab functionality to FireFox

1. Type about:support in url field of firefox, hit enter.
2. On the ‘Profile Folder’ setting (about 8th from the top) click the ‘Open Folder’ button.
3. Create the ‘chrome’ subfolder, if you don’t already have one.
4. Inside that folder create a file called ‘userChrome.css’ (right click > new > text document - make sure to name the file userChrome.css and not userChrome.css.txt or something)
5. Open the file for editing and paste the contents of this file;
   https://gist.githubusercontent.com/forexhill/b0446cc31e5001c9e87754df83f0f1ca/raw/f4f1a807d43434a1f10908364dec56ecdf08422c/gistfile1.txt
   Into it.
6. Restart FF and you should have multi-row tabs
7. Enjoy!

.tabbrowser-tab:not([pinned]) {min-width:35px;max-width:35px;}
.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;}

Here is updated code that works in FireFox v65 and newer.

.tabbrowser-tab:not([pinned]) { flex-grow:1; min-width:150px; } .tabbrowser-tab,.tab-background { height:40px; } .tab-stack { width: 100%; } #tabbrowser-tabs .scrollbox-innerbox { display: flex; flex-wrap: wrap; } #tabbrowser-tabs .arrowscrollbox-scrollbox { overflow: visible; display: block; } #main-window[sizemode=“maximized”] #TabsToolbar{ margin-left:var(–tab-min-height); } #tabbrowser-tabs .scrollbutton-up,#tabbrowser-tabs .scrollbutton-down,#alltabs-button,.tabbrowser-tab:not([fadein]){ display: none; }

This was taken from https://discourse.mozilla.org/t/tabs-in-two-or-more-rows-like-tabmixpro-in-quantum/21657/20