diff --git a/extensions/metrics/src/nsMetricsEventItem.h b/extensions/metrics/src/nsMetricsEventItem.h
index 18c71eab033..0baf3ab62af 100644
--- a/extensions/metrics/src/nsMetricsEventItem.h
+++ b/extensions/metrics/src/nsMetricsEventItem.h
@@ -42,6 +42,7 @@
 #include "nsIMetricsService.h"
 #include "nsCOMArray.h"
 #include "nsCOMPtr.h"
+#include "nsString.h"
 
 class nsIPropertyBag;