--- a/dom/bindings/BindingDeclarations.h
+++ b/dom/bindings/BindingDeclarations.h
@@ -438,16 +438,17 @@ public:
   // Make us work with smart-ptr helpers that expect a get()
   T* get() const {
     MOZ_ASSERT(inited);
     MOZ_ASSERT(ptr);
     return ptr;
   }
 
 protected:
+  // ptr is left uninitialized for optimization purposes.
   T* ptr;
 #ifdef DEBUG
   bool inited;
 #endif
 };
 
 // Class for representing sequences in arguments.  We use a non-auto array
 // because that allows us to use sequences of sequences and the like.  This

--- a/dom/bindings/FakeString.h
+++ b/dom/bindings/FakeString.h
@@ -105,17 +105,19 @@ operator const nsAString& () const {
     return *reinterpret_cast<const nsString*>(this);
   }
 
 private:
   nsAString* ToAStringPtr() {
     return reinterpret_cast<nsString*>(this);
   }
 
+  // mData is left uninitialized for optimization purposes.
   nsString::char_type* mData;
+  // mLength is left uninitialized for optimization purposes.
   nsString::size_type mLength;
   nsString::DataFlags mDataFlags;
   nsString::ClassFlags mClassFlags;
 
   static const size_t sInlineCapacity = 64;
   nsString::char_type mInlineStorage[sInlineCapacity];
 
   FakeString(const FakeString& other) = delete;