Index: wp-includes/nav-menu-template.php
===================================================================
--- wp-includes/nav-menu-template.php	(revision 15292)
+++ wp-includes/nav-menu-template.php	(working copy)
@@ -307,6 +307,8 @@
 
 	$possible_object_parents = array_filter( $possible_object_parents );
 
+	$front_page_url = home_url('/');
+
 	foreach ( (array) $menu_items as $key => $menu_item ) {
 		$classes = (array) $menu_item->classes;
 		$classes[] = 'menu-item';
@@ -370,6 +372,10 @@
 				$active_parent_item_ids[] = (int) $menu_item->menu_item_parent;
 				$active_parent_object_ids[] = (int) $menu_item->post_parent;
 				$active_object = $menu_item->object;
+
+			// give front page item current-menu-item class when extra query arguments involved
+			} elseif ( $item_url == $front_page_url && is_front_page() ) {
+				$classes[] = 'current-menu-item';
 			}
 		}