Bug 1391328 - Remove top border from tabs on OSX. r=nhnt11

MozReview-Commit-ID: B2dmIlJLM2j

--HG--
extra : rebase_source : be24d5c5ee3a96aef5d3b217e8014dd38cea8565

browser/themes/shared/tabs.inc.css

@@ -308,9 +308,11 @@
   background-clip: padding-box;
 }
 
+%ifdef MENUBAR_CAN_AUTOHIDE
 #toolbar-menubar:not([autohide=true]) ~ #TabsToolbar > #tabbrowser-tabs > .tabbrowser-tab > .tab-stack > .tab-background {
   border-top-style: solid;
 }
+%endif
 
 .tab-background[selected=true] {
   border-left-style: solid;