diff --git a/xpcom/base/nsDebug.cpp b/xpcom/base/nsDebug.cpp
index 4a079225cc65..82354d59c4a5 100644
--- a/xpcom/base/nsDebug.cpp
+++ b/xpcom/base/nsDebug.cpp
@@ -371,7 +371,7 @@ NS_CheckThreadSafe(void* owningThread, const char* msg)
     check = gCheckThreadSafeDefault || getenv("XPCOM_CHECK_THREADSAFE") != 0;
   }
   if (check) {
-    //NS_ASSERTION(owningThread == NS_CurrentThread(), msg);
+    NS_ASSERTION(owningThread == NS_CurrentThread(), msg);
   }
 }
 
diff --git a/xpcom/glue/nsDebug.cpp b/xpcom/glue/nsDebug.cpp
index 4a079225cc65..82354d59c4a5 100644
--- a/xpcom/glue/nsDebug.cpp
+++ b/xpcom/glue/nsDebug.cpp
@@ -371,7 +371,7 @@ NS_CheckThreadSafe(void* owningThread, const char* msg)
     check = gCheckThreadSafeDefault || getenv("XPCOM_CHECK_THREADSAFE") != 0;
   }
   if (check) {
-    //NS_ASSERTION(owningThread == NS_CurrentThread(), msg);
+    NS_ASSERTION(owningThread == NS_CurrentThread(), msg);
   }
 }