--- a/toolkit/components/telemetry/TelemetryHistogram.cpp
+++ b/toolkit/components/telemetry/TelemetryHistogram.cpp
@@ -494,17 +494,17 @@ internal_GetHistogramByEnumId(mozilla::T
     return NS_OK;
   }
 
   const HistogramInfo &p = gHistograms[id];
   if (p.keyed) {
     return NS_ERROR_FAILURE;
   }
 
-  nsCString histogramName;
+  nsAutoCString histogramName;
   histogramName.Append(p.id());
   if (const char* suffix = SuffixForProcessType(aProcessType)) {
     histogramName.AppendASCII(suffix);
   }
 
   nsresult rv = internal_HistogramGet(histogramName.get(), p.expiration(),
                                       p.histogramType, p.min, p.max,
                                       p.bucketCount, true, &h);
@@ -1398,17 +1398,17 @@ internal_AccumulateChildKeyed(GeckoProce
   const char* suffix = SuffixForProcessType(aProcessType);
   if (!suffix) {
     MOZ_ASSERT_UNREACHABLE("suffix should not be null");
     return;
   }
 
   const HistogramInfo& th = gHistograms[aId];
 
-  nsCString id;
+  nsAutoCString id;
   id.Append(th.id());
   id.AppendASCII(suffix);
 
   KeyedHistogram* keyed = internal_GetKeyedHistogramById(id);
   MOZ_ASSERT(keyed);
   keyed->Add(aKey, aSample);
 }