--- a/browser/themes/linux/browser.css
+++ b/browser/themes/linux/browser.css
@@ -582,17 +582,21 @@ html|span.ac-emphasize-text-url {
   color: -moz-FieldText;
 }
 
 #sidebar-header {
   border-bottom: 1px solid ThreeDShadow;
 }
 
 .sidebar-splitter {
-  border-color: ThreeDShadow;
+  -moz-appearance: none;
+  width: 6px;
+  background-color: -moz-dialog;
+  border: 1px ThreeDShadow;
+  border-style: none solid;
 }
 
 .browserContainer > findbar {
   background-color: -moz-dialog;
   color: -moz-DialogText;
   text-shadow: none;
 }
 

--- a/browser/themes/shared/sidebar.inc.css
+++ b/browser/themes/shared/sidebar.inc.css
@@ -6,34 +6,38 @@
 
 .sidebar-header,
 #sidebar-header {
   font-size: 1.333em;
   font-weight: lighter;
   padding: 8px;
 }
 
+%ifndef MOZ_WIDGET_GTK
+% We don't let the splitter overlap the sidebar on Linux since the sidebar's
+% scrollbar is too narrow on Linux.
 .sidebar-splitter {
   -moz-appearance: none;
   border: 0 solid;
   border-inline-end-width: 1px;
   min-width: 1px;
-  width: 8px;
+  width: 4px;
   background-image: none !important;
   background-color: transparent;
-  margin-inline-start: -8px;
+  margin-inline-start: -4px;
   position: relative;
 }
 
 #sidebar-box[positionend] + .sidebar-splitter {
   border-inline-end-width: 0;
   border-inline-start-width: 1px;
   margin-inline-start: 0;
-  margin-inline-end: -8px;
+  margin-inline-end: -4px;
 }
+%endif
 
 #sidebar-throbber[loading="true"] {
   list-style-image: url("chrome://global/skin/icons/loading.png");
 }
 
 @media (min-resolution: 2dppx) {
   .sidebar-throbber[loading="true"],
   #sidebar-throbber[loading="true"] {