I've narrowed my tabs with userchrome.css so that they look like this. I think you could make the min-width even smaller so that the scrolling never happens, but then the tab icons won't be visible.
Can't bother debugging my userchrome right now to give a reproducible example but I think this is the relevant part:
.tabbrowser-tab[fadein]:not([pinned]) {
min-width: 30px !important;
padding-inline: 0 !important; /* not sure why I added this */
}