--- a/devtools/client/netmonitor/src/assets/styles/RequestList.css
+++ b/devtools/client/netmonitor/src/assets/styles/RequestList.css
@@ -105,16 +105,18 @@
   padding: 0;
 }
 
 .requests-list-headers {
   display: table-header-group;
   height: 24px;
   padding: 0;
   width: 100%;
+  border-right-width: 1px;
+  border-right-color: var(--theme-splitter-color);
 }
 
 .requests-list-headers .requests-list-column:first-child .requests-list-header-button {
   border-width: 0;
 }
 
 .requests-list-header-button {
   background-color: transparent;