Bug 1570996 - Removed the bottom border from the last message in the Whats New Panel r=k88hudson

Differential Revision: https://phabricator.services.mozilla.com/D42720

--HG--
extra : moz-landing-system : lando

browser/themes/shared/customizableui/panelUI.inc.css

@@ -1864,6 +1864,10 @@ toolbarpaletteitem[place="menu-panel"] > .subviewbutton-nav::after {
   background: var(--panel-separator-color);
 }
 
+#PanelUI-whatsNew .whatsNew-message:last-child {
+  border-bottom: none;
+}
+
 #PanelUI-whatsNew .whatsNew-message-date {
   font-size: 11px;
   margin: 0 -12px;