--- a/xpcom/string/public/nsStringBuffer.h
+++ b/xpcom/string/public/nsStringBuffer.h
@@ -164,11 +164,16 @@ class nsStringBuffer
        * NOTE: storage size is measured in bytes even for wide strings;
        *       however, string length is always measured in storage units
        *       (2-byte units for wide strings).
        */
       void ToString(PRUint32 len, nsAString &str,
                            bool aMoveOwnership = false);
       void ToString(PRUint32 len, nsACString &str,
                            bool aMoveOwnership = false);
+
+      /**
+       * This measures the size only if the StringBuffer is unshared.
+       */
+      size_t SizeOfIncludingThisIfUnshared(nsMallocSizeOfFun aMallocSizeOf) const;
   };
 
 #endif /* !defined(nsStringBuffer_h__ */

--- a/xpcom/string/src/nsSubstring.cpp
+++ b/xpcom/string/src/nsSubstring.cpp
@@ -302,16 +302,26 @@ nsStringBuffer::ToString(PRUint32 len, n
     flags = (flags & 0xFFFF0000) | nsCSubstring::F_SHARED | nsCSubstring::F_TERMINATED;
 
     if (!aMoveOwnership) {
       AddRef();
     }
     accessor->set(data, len, flags);
   }
 
+size_t
+nsStringBuffer::SizeOfIncludingThisIfUnshared(nsMallocSizeOfFun aMallocSizeOf) const
+  {
+    if (!IsReadonly())
+      {
+        return aMallocSizeOf(this, sizeof(nsStringBuffer) + mStorageSize);
+      }
+    return 0;
+  }
+
 // ---------------------------------------------------------------------------
 
 
   // define nsSubstring
 #include "string-template-def-unichar.h"
 #include "nsTSubstring.cpp"
 #include "string-template-undef.h"