--- a/dom/base/nsJSEnvironment.cpp
+++ b/dom/base/nsJSEnvironment.cpp
@@ -3491,22 +3491,22 @@ DOMGCSliceCallback(JSRuntime *aRt, js::G
     PRTime now = PR_Now();
     PRTime delta = 0;
     if (sFirstCollectionTime) {
       delta = now - sFirstCollectionTime;
     } else {
       sFirstCollectionTime = now;
     }
 
-    NS_NAMED_LITERAL_STRING(kFmt, "GC(T+%.1f) %s");
-    nsString msg, gcstats;
+    NS_NAMED_LITERAL_STRING(kFmt, "GC(T+%.1f) ");
+    nsString prefix, gcstats;
     gcstats.Adopt(aDesc.formatMessage(aRt));
-    msg.Adopt(nsTextFormatter::smprintf(kFmt.get(),
-                                        double(delta) / PR_USEC_PER_SEC,
-                                        gcstats.get()));
+    prefix.Adopt(nsTextFormatter::smprintf(kFmt.get(),
+                                           double(delta) / PR_USEC_PER_SEC));
+    nsString msg = prefix + gcstats;
     nsCOMPtr<nsIConsoleService> cs = do_GetService(NS_CONSOLESERVICE_CONTRACTID);
     if (cs) {
       cs->LogStringMessage(msg.get());
     }
   }
 
   // Prevent cycle collections and shrinking during incremental GC.
   if (aProgress == js::GC_CYCLE_BEGIN) {