diff --git a/toolkit/components/build/nsToolkitCompsCID.h b/toolkit/components/build/nsToolkitCompsCID.h
index b358b00447ed..ef2d0cac57f6 100644
--- a/toolkit/components/build/nsToolkitCompsCID.h
+++ b/toolkit/components/build/nsToolkitCompsCID.h
@@ -163,9 +163,9 @@
 #define NS_TYPEAHEADFIND_CID \
 { 0xe7f70966, 0x9a37, 0x48d7, { 0x8a, 0xeb, 0x35, 0x99, 0x8f, 0x31, 0x09, 0x0e} }
 
-// {51464459-4e46-4f31-8745-4acfa7c1e2f2}
+// {7902e243-9b00-4673-9288-1b7fc246a8f8}
 #define NS_URLCLASSIFIERPREFIXSET_CID \
-{ 0x51464459, 0x4e46, 0x4f31, { 0x87, 0x45, 0x4a, 0xcf, 0xa7, 0xc1, 0xe2, 0xf2} }
+{ 0x7902e243, 0x9b00, 0x4673, { 0x92, 0x88, 0x1b, 0x7f, 0xc2, 0x46, 0xa8, 0xf8} }
 
 // {5eb7c3c1-ec1f-4007-87cc-eefb37d68ce6}
 #define NS_URLCLASSIFIERDBSERVICE_CID \
diff --git a/toolkit/components/url-classifier/nsIUrlClassifierPrefixSet.idl b/toolkit/components/url-classifier/nsIUrlClassifierPrefixSet.idl
index eb2e241121bc..c8c53d4ebf1f 100644
--- a/toolkit/components/url-classifier/nsIUrlClassifierPrefixSet.idl
+++ b/toolkit/components/url-classifier/nsIUrlClassifierPrefixSet.idl
@@ -41,7 +41,7 @@
 
 interface nsIArray;
 
-[scriptable, uuid(51464459-4e46-4f31-8745-4acfa7c1e2f2)]
+[scriptable, uuid(7902e243-9b00-4673-9288-1b7fc246a8f8)]
 interface nsIUrlClassifierPrefixSet : nsISupports
 {
   void setPrefixes([const, array, size_is(aLength)] in unsigned long aPrefixes,
@@ -51,7 +51,6 @@ interface nsIUrlClassifierPrefixSet : nsISupports
   boolean contains(in unsigned long aPrefix);
   boolean probe(in unsigned long aPrefix, in unsigned long aKey,
                 inout boolean aReady);
-  PRUint32 sizeOfIncludingThis();
   PRUint32 getKey();
   boolean isEmpty();
   void loadFromFile(in nsIFile aFile);
diff --git a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
index 7f46f9db9634..e4f0eff03631 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
@@ -3691,10 +3691,8 @@ nsUrlClassifierDBServiceWorker::LoadPrefixSet(nsCOMPtr<nsIFile> & aFile)
   }
 
 #ifdef DEBUG
-  PRUint32 size = 0;
-  rv = mPrefixSet->SizeOfIncludingThis(&size);
-  LOG(("SB tree done, size = %d bytes\n", size));
-  NS_ENSURE_SUCCESS(rv, rv);
+  LOG(("SB tree done, size = %d bytes\n",
+       mPrefixSet->SizeOfIncludingThis(moz_malloc_size_of)));
 #endif
 #if defined(PR_LOGGING)
   if (LOG_ENABLED()) {
diff --git a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
index 339e2b07de42..d3d14210fd7c 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp
@@ -84,6 +84,9 @@ private:
 
 NS_IMPL_THREADSAFE_ISUPPORTS1(nsPrefixSetReporter, nsIMemoryReporter)
 
+NS_MEMORY_REPORTER_MALLOC_SIZEOF_FUN(StoragePrefixSetMallocSizeOf,
+                                     "storage/prefixset")
+
 nsPrefixSetReporter::nsPrefixSetReporter(nsUrlClassifierPrefixSet * aParent,
                                          const nsACString & aName)
 : mParent(aParent)
@@ -126,10 +129,8 @@ nsPrefixSetReporter::GetUnits(PRInt32 * aUnits)
 NS_IMETHODIMP
 nsPrefixSetReporter::GetAmount(PRInt64 * aAmount)
 {
-  PRUint32 size;
-  nsresult rv = mParent->SizeOfIncludingThis(&size);
-  *aAmount = size;
-  return rv;
+  *aAmount = mParent->SizeOfIncludingThis(StoragePrefixSetMallocSizeOf);
+  return NS_OK;
 }
 
 NS_IMETHODIMP
@@ -333,16 +334,16 @@ nsUrlClassifierPrefixSet::Contains(PRUint32 aPrefix, bool * aFound)
   return NS_OK;
 }
 
-NS_IMETHODIMP
-nsUrlClassifierPrefixSet::SizeOfIncludingThis(PRUint32 * aSize)
+size_t
+nsUrlClassifierPrefixSet::SizeOfIncludingThis(nsMallocSizeOfFun mallocSizeOf)
 {
   MutexAutoLock lock(mPrefixSetLock);
-  size_t usable = moz_malloc_usable_size(this);
-  *aSize = (PRUint32)(usable ? usable : sizeof(*this));
-  *aSize += mDeltas.SizeOf();
-  *aSize += mIndexPrefixes.SizeOf();
-  *aSize += mIndexStarts.SizeOf();
-  return NS_OK;
+  size_t n = 0;
+  n += mallocSizeOf(this, sizeof(nsUrlClassifierPrefixSet));
+  n += mDeltas.SizeOf();
+  n += mIndexPrefixes.SizeOf();
+  n += mIndexStarts.SizeOf();
+  return n;
 }
 
 NS_IMETHODIMP
diff --git a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.h b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.h
index ec828d7d4a17..23cf105869d3 100644
--- a/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.h
+++ b/toolkit/components/url-classifier/nsUrlClassifierPrefixSet.h
@@ -70,9 +70,6 @@ public:
   // if aReady is set, we will block until there are any entries
   // if not set, we will return in aReady whether we were ready or not
   NS_IMETHOD Probe(PRUint32 aPrefix, PRUint32 aKey, bool* aReady, bool* aFound);
-  // Return the estimated size of the set on disk and in memory,
-  // in bytes
-  NS_IMETHOD SizeOfIncludingThis(PRUint32* aSize);
   NS_IMETHOD IsEmpty(bool * aEmpty);
   NS_IMETHOD LoadFromFile(nsIFile* aFile);
   NS_IMETHOD StoreToFile(nsIFile* aFile);
@@ -81,6 +78,10 @@ public:
 
   NS_DECL_ISUPPORTS
 
+  // Return the estimated size of the set on disk and in memory,
+  // in bytes
+  size_t SizeOfIncludingThis(nsMallocSizeOfFun mallocSizeOf);
+
 protected:
   static const PRUint32 DELTAS_LIMIT = 100;
   static const PRUint32 MAX_INDEX_DIFF = (1 << 16);