--- a/mobile/android/base/java/org/mozilla/gecko/tabs/TabsGridLayout.java
+++ b/mobile/android/base/java/org/mozilla/gecko/tabs/TabsGridLayout.java
@@ -45,13 +45,14 @@ abstract class TabsGridLayout extends Ta
     @Override
     protected boolean addAtIndexRequiresScroll(int index) {
         final GridLayoutManager layoutManager = (GridLayoutManager) getLayoutManager();
         final int spanCount = layoutManager.getSpanCount();
         final int firstVisibleIndex = layoutManager.findFirstVisibleItemPosition();
         // When you add an item at the first visible position to a GridLayoutManager and there's
         // room to scroll, RecyclerView scrolls the new position to anywhere from near the bottom of
         // its row to completely offscreen (for unknown reasons), so we need to scroll to fix that.
-        // We also scroll when the item being added is the only item on the final row.
-        return index == firstVisibleIndex ||
-                (index == getAdapter().getItemCount() - 1 && index % spanCount == 0);
+        // We also always need to scroll if the new tab is a new last tab on a row by itself; more
+        // generally, another app can open a new last tab with the tabs tray open and the scroll at
+        // an arbitrary position, so we need to always scroll in that more general case as well.
+        return index == firstVisibleIndex || index == getAdapter().getItemCount() - 1;
     }
 }

--- a/mobile/android/base/java/org/mozilla/gecko/tabs/TabsLayout.java
+++ b/mobile/android/base/java/org/mozilla/gecko/tabs/TabsLayout.java
@@ -100,17 +100,19 @@ public abstract class TabsLayout extends
     @Override
     public void onTabChanged(Tab tab, Tabs.TabEvents msg, String data) {
         if (msg != Tabs.TabEvents.RESTORED && tab.getType() != type) {
             return;
         }
 
         switch (msg) {
             case ADDED:
-                final int tabIndex = Integer.parseInt(data);
+                int tabIndex = Integer.parseInt(data);
+                // A tabIndex of -1 means "add a new last tab".
+                tabIndex = tabIndex == -1 ? tabsAdapter.getItemCount() : tabIndex;
                 tabsAdapter.notifyTabInserted(tab, tabIndex);
                 if (addAtIndexRequiresScroll(tabIndex)) {
                     // (The SELECTED tab is updated *after* this call to ADDED, so don't just call
                     // updateSelectedPosition().)
                     scrollToPosition(tabIndex);
                 }
                 break;
 
@@ -127,18 +129,19 @@ public abstract class TabsLayout extends
             case RECORDING_CHANGE:
             case AUDIO_PLAYING_CHANGE:
                 tabsAdapter.notifyTabChanged(tab);
                 break;
         }
     }
 
     /**
-     * Addition of a tab at selected positions (dependent on LayoutManager) will result in a tab
-     * being added out of view - return true if {@code index} is such a position.
+     * Addition of a tab at selected positions (dependent on LayoutManager) can result in a tab
+     * being added out of view - return true if {@code index} is such a position.  This should be
+     * called only after the add has occurred.
      */
     abstract protected boolean addAtIndexRequiresScroll(int index);
 
     protected int getSelectedAdapterPosition() {
         return tabsAdapter.getPositionForTab(Tabs.getInstance().getSelectedTab());
     }
 
     @Override

--- a/mobile/android/base/java/org/mozilla/gecko/tabs/TabsLayoutAdapter.java
+++ b/mobile/android/base/java/org/mozilla/gecko/tabs/TabsLayoutAdapter.java
@@ -79,28 +79,29 @@ public class TabsLayoutAdapter
 
     /* package */ void notifyTabChanged(Tab tab) {
         final int position = getPositionForTab(tab);
         if (position != -1) {
             notifyItemChanged(position);
         }
     }
 
+    /**
+     * Insert {@code tab} in the tabs list at the specified {@code index}.
+     * @param index 0 <= index <= current tab count
+     */
     /* package */ void notifyTabInserted(Tab tab, int index) {
         if (index >= 0 && index <= tabs.size()) {
             tabs.add(index, tab);
             notifyItemInserted(index);
         } else {
-            // Add to the end.
+            // The index is out of bounds; add to the end.
             tabs.add(tab);
             notifyItemInserted(tabs.size() - 1);
-            // index == -1 is a valid way to add to the end, the other cases are errors.
-            if (index != -1) {
-                Log.e(LOGTAG, "Tab was inserted at an invalid position: " + Integer.toString(index));
-            }
+            Log.e(LOGTAG, "Tab was inserted at an invalid position: " + Integer.toString(index));
         }
     }
 
     /* package */ boolean moveTab(int fromPosition, int toPosition) {
         final int fromTabId = tabs.get(fromPosition).getId();
         final int toTabId = tabs.get(toPosition).getId();
         JavaUtil.moveInList(tabs, fromPosition, toPosition);
         notifyItemMoved(fromPosition, toPosition);

--- a/mobile/android/base/java/org/mozilla/gecko/tabs/TabsListLayout.java
+++ b/mobile/android/base/java/org/mozilla/gecko/tabs/TabsListLayout.java
@@ -106,11 +106,14 @@ public class TabsListLayout extends Tabs
             }, cascadeDelay);
 
             cascadeDelay += ANIMATION_CASCADE_DELAY;
         }
     }
 
     @Override
     protected boolean addAtIndexRequiresScroll(int index) {
+        // Scroll if we're adding a new first tab (from a close undo) or if we're adding a new last
+        // tab (needed both for close undo and for when a new last tab is added by another app
+        // opening a link in Fennec where Fennec loads with the tabs tray already open).
         return index == 0 || index == getAdapter().getItemCount() - 1;
     }
 }