diff --git a/toolkit/components/places/nsAndroidHistory.cpp b/toolkit/components/places/nsAndroidHistory.cpp
index 5faccde4541c..ee445269395f 100644
--- a/toolkit/components/places/nsAndroidHistory.cpp
+++ b/toolkit/components/places/nsAndroidHistory.cpp
@@ -72,7 +72,7 @@ nsAndroidHistory::RegisterVisitedCallback(nsIURI *aURI, Link *aContent)
   nsCAutoString uri;
   nsresult rv = aURI->GetSpec(uri);
   if (NS_FAILED(rv)) return rv;
-  nsString uriString = NS_ConvertUTF8toUTF16(uri);
+  NS_ConvertUTF8toUTF16 uriString(uri);
 
   nsTArray<Link*>* list = mListeners.Get(uriString);
   if (! list) {
@@ -98,7 +98,7 @@ nsAndroidHistory::UnregisterVisitedCallback(nsIURI *aURI, Link *aContent)
   nsCAutoString uri;
   nsresult rv = aURI->GetSpec(uri);
   if (NS_FAILED(rv)) return rv;
-  nsString uriString = NS_ConvertUTF8toUTF16(uri);
+  NS_ConvertUTF8toUTF16 uriString(uri);
 
   nsTArray<Link*>* list = mListeners.Get(uriString);
   if (! list)
@@ -126,7 +126,7 @@ nsAndroidHistory::VisitURI(nsIURI *aURI, nsIURI *aLastVisitedURI, PRUint32 aFlag
     nsCAutoString uri;
     nsresult rv = aURI->GetSpec(uri);
     if (NS_FAILED(rv)) return rv;
-    nsString uriString = NS_ConvertUTF8toUTF16(uri);
+    NS_ConvertUTF8toUTF16 uriString(uri);
     bridge->MarkURIVisited(uriString);
   }
   return NS_OK;
diff --git a/toolkit/components/places/nsNavHistoryQuery.cpp b/toolkit/components/places/nsNavHistoryQuery.cpp
index 2578290df3e3..6ddd9a4c16cc 100644
--- a/toolkit/components/places/nsNavHistoryQuery.cpp
+++ b/toolkit/components/places/nsNavHistoryQuery.cpp
@@ -791,7 +791,7 @@ nsNavHistory::TokensToQueries(const nsTArray<QueryKeyValuePair>& aTokens,
     } else if (kvp.key.EqualsLiteral(QUERYKEY_TAG)) {
       nsCAutoString unescaped(kvp.value);
       NS_UnescapeURL(unescaped); // modifies input
-      nsString tag = NS_ConvertUTF8toUTF16(unescaped);
+      NS_ConvertUTF8toUTF16 tag(unescaped);
       if (!tags.Contains(tag)) {
         NS_ENSURE_TRUE(tags.AppendElement(tag), NS_ERROR_OUT_OF_MEMORY);
       }