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tab_separator_lines.css

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+/* 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-end: 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%
+  );
+}
+
+.tabbrowser-tab:hover,
+.tabbrowser-tab[beforehovered],
+.tabbrowser-tab[selected],
+.tabbrowser-tab[last-visible-tab],
+.tabbrowser-tab[beforeselected-visible]{ border-image: none !important; }