--- a/layout/generic/nsTextRunTransformations.cpp
+++ b/layout/generic/nsTextRunTransformations.cpp
@@ -281,18 +281,18 @@ nsCaseTransformTextRunFactory::Transform
     nsTArray<bool>& aCharsToMergeArray,
     nsTArray<bool>& aDeletedCharsArray,
     const nsTransformedTextRun* aTextRun,
     uint32_t aOffsetInTextRun,
     nsTArray<uint8_t>* aCanBreakBeforeArray,
     nsTArray<RefPtr<nsTransformedCharStyle>>* aStyleArray)
 {
   bool auxiliaryOutputArrays = aCanBreakBeforeArray && aStyleArray;
-  NS_PRECONDITION(!auxiliaryOutputArrays || aTextRun,
-                  "text run must be provided to use aux output arrays");
+  MOZ_ASSERT(!auxiliaryOutputArrays || aTextRun,
+      "text run must be provided to use aux output arrays");
 
   uint32_t length = aString.Length();
   const char16_t* str = aString.BeginReading();
 
   bool mergeNeeded = false;
 
   bool capitalizeDutchIJ = false;
   bool prevIsLetter = false;