diff --git a/toolkit/themes/osx/global/nativescrollbars.css b/toolkit/themes/osx/global/nativescrollbars.css
index 82ef4d2ac1de..461634c9671b 100644
--- a/toolkit/themes/osx/global/nativescrollbars.css
+++ b/toolkit/themes/osx/global/nativescrollbars.css
@@ -41,14 +41,14 @@ slider[orient="vertical"] {
 
 /* ..... thumb ..... */
 
-thumb {
-  -moz-appearance: scrollbarthumb-horizontal;
-}
-
 thumb[orient="vertical"] {
   -moz-appearance: scrollbarthumb-vertical;
 }
 
+thumb[orient="horizontal"] {
+  -moz-appearance: scrollbarthumb-horizontal;
+}
+
 /* ..... increment ..... */
 
 scrollbarbutton[type="increment"] {
diff --git a/toolkit/themes/windows/global/xulscrollbars.css b/toolkit/themes/windows/global/xulscrollbars.css
index b11801d1fbc3..d04df0e15ce0 100644
--- a/toolkit/themes/windows/global/xulscrollbars.css
+++ b/toolkit/themes/windows/global/xulscrollbars.css
@@ -44,7 +44,7 @@ slider[orient="vertical"] {
 
 /* ::::: thumb (horizontal) ::::: */
 
-thumb {
+thumb[orient="vertical"] {
   -moz-appearance: scrollbarthumb-vertical;
   min-height: 8px;
 }