--- a/xpcom/threads/MozPromise.h
+++ b/xpcom/threads/MozPromise.h
@@ -921,17 +921,19 @@ protected:
   };
 
   const char* mCreationSite; // For logging
   Mutex mMutex;
   ResolveOrRejectValue mValue;
 #ifdef PROMISE_DEBUG
   uint32_t mMagic1 = sMagic;
 #endif
-  nsTArray<RefPtr<ThenValueBase>> mThenValues;
+  // Try shows we never have more than 3 elements when IsExclusive is false.
+  // So '3' is a good value to avoid heap allocation in most cases.
+  AutoTArray<RefPtr<ThenValueBase>, IsExclusive ? 1 : 3> mThenValues;
 #ifdef PROMISE_DEBUG
   uint32_t mMagic2 = sMagic;
 #endif
   nsTArray<RefPtr<Private>> mChainedPromises;
 #ifdef PROMISE_DEBUG
   uint32_t mMagic3 = sMagic;
 #endif
   bool mHaveRequest;