--- a/dom/media/MediaData.h
+++ b/dom/media/MediaData.h
@@ -640,30 +640,30 @@ public:
   size_t Size();
   // Writeable reference to MediaRawData::mCryptoInternal
   CryptoSample& mCrypto;
 
   // Data manipulation methods. mData and mSize may be updated accordingly.
 
   // Set size of buffer, allocating memory as required.
   // If size is increased, new buffer area is filled with 0.
-  bool SetSize(size_t aSize);
+  MOZ_MUST_USE bool SetSize(size_t aSize);
   // Add aData at the beginning of buffer.
-  bool Prepend(const uint8_t* aData, size_t aSize);
+  MOZ_MUST_USE bool Prepend(const uint8_t* aData, size_t aSize);
   // Replace current content with aData.
-  bool Replace(const uint8_t* aData, size_t aSize);
+  MOZ_MUST_USE bool Replace(const uint8_t* aData, size_t aSize);
   // Clear the memory buffer. Will set target mData and mSize to 0.
   void Clear();
   // Remove aSize bytes from the front of the sample.
   void PopFront(size_t aSize);
 
 private:
   friend class MediaRawData;
   explicit MediaRawDataWriter(MediaRawData* aMediaRawData);
-  bool EnsureSize(size_t aSize);
+  MOZ_MUST_USE bool EnsureSize(size_t aSize);
   MediaRawData* mTarget;
 };
 
 class MediaRawData : public MediaData
 {
 public:
   MediaRawData();
   MediaRawData(const uint8_t* aData, size_t aSize);