--- a/toolkit/themes/shared/in-content/common.inc.css
+++ b/toolkit/themes/shared/in-content/common.inc.css
@@ -130,21 +130,16 @@ xul|tabpanels {
 
 xul|tabs {
   margin-bottom: 15px;
   border-top: 1px solid var(--in-content-box-border-color);
   border-bottom: 1px solid var(--in-content-box-border-color);
   background-color: var(--in-content-page-background);
 }
 
-xul|*.tabs-left,
-xul|*.tabs-right {
-  border-bottom: none;
-}
-
 xul|tab {
   -moz-appearance: none;
   margin-top: 0;
   padding: 4px 20px;
   min-height: 44px;
   color: var(--in-content-tab-color);
   background-color: var(--in-content-page-background);
   border-width: 0;

--- a/toolkit/themes/windows/global/tabbox.css
+++ b/toolkit/themes/windows/global/tabbox.css
@@ -3,24 +3,16 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 /* ===== tabbox.css =================================================
   == Styles used by XUL tab-related elements.
   ======================================================================= */
 
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 
-/* ::::: tabs ::::: */
-
-.tabs-left,
-.tabs-right {
-  border-bottom: 2px solid;
-  -moz-border-bottom-colors: ThreeDHighlight ThreeDLightShadow;
-}
-
 /* ::::: tabpanels ::::: */
 
 tabpanels {
   -moz-appearance: tabpanels;
   padding: 8px;
   color: -moz-DialogText;
 }
 
@@ -63,15 +55,8 @@ tab:first-of-type[visuallyselected="true
   padding: 2px 4px 1px 4px;
 }
 
 .tab-bottom[visuallyselected="true"] {
   margin-bottom: 0;
   padding: 4px 6px 1px 6px;
 }
 
-/* ::::: tabs-bottom ::::: */
-
-.tabs-bottom > .tabs-left,
-.tabs-bottom > .tabs-right {
-  border-top: 1px solid ThreeDShadow;
-  border-bottom: none;
-}