wrap the fx71 selectors in @moz-document

chrome/multi-row_tabs.css

@@ -20,17 +20,19 @@
 }
 /* Selectors for Firefox 71+ */
 /* These are not tabs toolbar specific but horizontal scrollbox isn't used elsewhere, except in bookmarks toolbar but there it doesn't have [part] attribute since it's not in shadow-root */
-.scrollbutton-up[orient="horizontal"][part]~spacer,
+@-moz-document url(chrome://browser/content/browser.xhtml){
-.scrollbutton-up[orient="horizontal"][part],
+  .scrollbutton-up[orient="horizontal"][part]~spacer,
-.scrollbutton-down[orient="horizontal"][part]{ display: none }
+  .scrollbutton-up[orient="horizontal"][part],
+  .scrollbutton-down[orient="horizontal"][part]{ display: none }
 
-scrollbox[part][orient="horizontal"]{
+  scrollbox[part][orient="horizontal"]{
-  display: flex;
+    display: flex;
-  flex-wrap: wrap;
+    flex-wrap: wrap;
-  overflow-y: auto;
+    overflow-y: auto;
-  max-height: calc(var(--tab-min-height) * var(--multirow-n-rows));
+    max-height: calc(var(--tab-min-height) * var(--multirow-n-rows));
-  scrollbar-color: currentColor transparent;
+    scrollbar-color: currentColor transparent;
-  scrollbar-width: thin;
+    scrollbar-width: thin;
+  }
 }
 
 /* Test for Firefox > 66 */