Shame on you Firefox staff, defecting purposely this very important functionality !
I find myself the solution here:
https://techne.alaya.net/?p=19043
Basically, make an empty file: userChrome.css and move it here:
C:\Users\Teo\AppData\Roaming\Mozilla\Firefox\Profiles\mqqqqqre.default-release\chrome\
Then make this configuration inside FF and restart:
about:config
toolkit.legacyUserProfileCustomizations.stylesheets
true //if it’s set to false it means userChrome.css will be ignored.
userChrome.css content :
(just copy/paste this code inside your empty file)
/* Makes tabs to appear on multiple lines */
/* 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 */
/* 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. */
}
/* Scrollbar can't be clicked but the rows can be scrolled with mouse wheel */
/* Uncomment the next line if you want to be able to use the scrollbar with mouse clicks */
/* #tabbrowser-arrowscrollbox{ -moz-window-dragging: no-drag } */
/* Uncommenting the above makes you unable to drag the window from empty space in the tab strip but normal draggable spaces will continue to work */
#tabbrowser-tabs{
min-height: unset !important;
padding-inline-start: 0px !important
}
@-moz-document url(chrome://browser/content/browser.xhtml){
#scrollbutton-up~spacer,
#scrollbutton-up,
#scrollbutton-down{ display: var(--scrollbutton-display-model,initial) }
scrollbox[part][orient="horizontal"]{
display: flex;
flex-wrap: wrap;
overflow-y: auto;
max-height: calc(var(--tab-min-height) * var(--multirow-n-rows));
scrollbar-color: currentColor transparent;
scrollbar-width: thin;
}
}
#tabbrowser-arrowscrollbox{
overflow: -moz-hidden-unscrollable;
display: block;
--scrollbutton-display-model: none;
}
.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% }
#alltabs-button,
:root:not([customizing]) #TabsToolbar #new-tab-button,
#tabbrowser-tabs spacer,
.tabbrowser-tab::after{ display: none !important }
