Bug #304042 --> Add separator line under "Check for updates" like in firefox

mail/base/content/mailWindowOverlay.xul

@@ -1717,6 +1717,9 @@
 #endif
           <menuitem id="checkForUpdates" accesskey="&updateCmd.accesskey;" 
                     label="&updateCmd.label;" oncommand="checkForUpdates();"/>
+#ifndef XP_MACOSX
+          <menuseparator/>
+#endif
           <menuitem id="aboutName" accesskey="&aboutCmd.accesskey;"
                     label="&aboutCmd.label;"
 #ifdef XP_MACOSX