Bug 1913279 - Fix vertical tabs issue with selected tab not scrolling on overflow r=tabbrowser-reviewers,desktop-theme-reviewers,dao

* Remove condition in tabs.js that prohibited overflow attribute being added for vertical orientation
* Remove redundant overflow-y:auto for #tabbrowser-arrowscrollbox and set a min-height for vertical tabs

Differential Revision: https://phabricator.services.mozilla.com/D221133

browser/components/tabbrowser/content/tabs.js

@@ -1148,13 +1148,8 @@
     }
 
     on_overflow(event) {
-      // Ignore overflow events:
-      // - from nested scrollable elements
-      // - for vertical orientation
-      if (
-        event.target != this.arrowScrollbox ||
-        event.originalTarget.getAttribute("orient") == "vertical"
-      ) {
+      // Ignore overflow events from nested scrollable elements
+      if (event.target != this.arrowScrollbox) {
         return;
       }