diff --git a/widget/gtk/gtk3drawing.cpp b/widget/gtk/gtk3drawing.cpp
index 6702ff95c8c1..2a481d70b587 100644
--- a/widget/gtk/gtk3drawing.cpp
+++ b/widget/gtk/gtk3drawing.cpp
@@ -2644,10 +2644,11 @@ GetScrollbarMetrics(GtkOrientation aOrientation)
     // button
     if (hasButtons) {
         metrics->size.button = GetMinMarginBox(MOZ_GTK_SCROLLBAR_BUTTON);
-        // If the buttons will cause Gecko to expand the track to fill
-        // available breadth, then add to the track border to prevent Gecko
-        // from expanding the thumb to fill available breadth.
         if (aOrientation == GTK_ORIENTATION_HORIZONTAL) {
+            metrics->size.button.Rotate();
+            // If the buttons will cause Gecko to expand the track to fill
+            // available breadth, then add to the track border to prevent Gecko
+            // from expanding the thumb to fill available breadth.
             gint extra = metrics->size.button.height - trackSizeForThumb.height;
             if (extra > 0) {
                 // If extra is odd, then the thumb is 0.5 pixels above
diff --git a/widget/gtk/gtkdrawing.h b/widget/gtk/gtkdrawing.h
index 1d9142c91b4b..521ed05e09c7 100644
--- a/widget/gtk/gtkdrawing.h
+++ b/widget/gtk/gtkdrawing.h
@@ -52,6 +52,12 @@ struct MozGtkSize {
     gint resultHeight = height + aBorder.top + aBorder.bottom;
     return {resultWidth, resultHeight};
   }
+  void Rotate()
+  {
+    gint tmp = width;
+    width = height;
+    height = tmp;
+  }
 };
 
 typedef struct {