tab_separator_lines.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/tab_separator_lines.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* Bring back tab separator lines that were removed in Proton */

.tabbrowser-tab{
  border-inline-start: 1px solid transparent !important;
  border-image: 0 1 linear-gradient(
    transparent 20%,
    color-mix(in srgb, currentColor 20%, transparent) 20%,
    color-mix(in srgb, currentColor 20%, transparent) 80%,
    transparent 80%
  ) !important;
}

.tabbrowser-tab:hover,
#tabbrowser-tabs:not([movingtab]) .tabbrowser-tab:hover + .tabbrowser-tab:not([first-visible-unpinned-tab]),
.tabbrowser-tab:first-child,
.tabbrowser-tab[selected],
.tabbrowser-tab[multiselected],
#tabbrowser-arrowscrollbox[overflowing] > .tabbrowser-tab[first-visible-unpinned-tab],
#tabbrowser-tabs:not([movingtab]) .tabbrowser-tab[multiselected] + .tabbrowser-tab,
#tabbrowser-tabs:not([movingtab]) .tabbrowser-tab[selected] + .tabbrowser-tab { border-image: none !important; }