--- a/modules/libpref/Preferences.cpp
+++ b/modules/libpref/Preferences.cpp
@@ -300,28 +300,30 @@ struct PrefsSizes
 {
   PrefsSizes()
     : mHashTable(0)
     , mPrefValues(0)
     , mStringValues(0)
     , mCacheData(0)
     , mRootBranches(0)
     , mPrefNameArena(0)
-    , mCallbacks(0)
+    , mCallbacksObjects(0)
+    , mCallbacksDomains(0)
     , mMisc(0)
   {
   }
 
   size_t mHashTable;
   size_t mPrefValues;
   size_t mStringValues;
   size_t mCacheData;
   size_t mRootBranches;
   size_t mPrefNameArena;
-  size_t mCallbacks;
+  size_t mCallbacksObjects;
+  size_t mCallbacksDomains;
   size_t mMisc;
 };
 }
 
 static ArenaAllocator<8192, 1> gPrefNameArena;
 
 class Pref
 {
@@ -753,16 +755,22 @@ struct CallbackNode
     : mDomain(moz_xstrdup(aDomain))
     , mFunc(aFunc)
     , mData(aData)
     , mMatchKind(aMatchKind)
     , mNext(nullptr)
   {
   }
 
+  void AddSizeOfIncludingThis(MallocSizeOf aMallocSizeOf, PrefsSizes& aSizes)
+  {
+    aSizes.mCallbacksObjects += aMallocSizeOf(this);
+    aSizes.mCallbacksDomains += aMallocSizeOf(mDomain.get());
+  }
+
   UniqueFreePtr<const char> mDomain;
 
   // If someone attempts to remove the node from the callback list while
   // NotifyCallbacks() is running, |func| is set to nullptr. Such nodes will
   // be removed at the end of NotifyCallbacks().
   PrefChangedFunc mFunc;
   void* mData;
   Preferences::MatchKind mMatchKind;
@@ -2693,18 +2701,17 @@ PreferenceServiceReporter::CollectReport
     for (uint32_t i = 0, count = gCacheData->Length(); i < count; ++i) {
       sizes.mCacheData += mallocSizeOf((*gCacheData)[i]);
     }
   }
 
   sizes.mPrefNameArena += gPrefNameArena.SizeOfExcludingThis(mallocSizeOf);
 
   for (CallbackNode* node = gFirstCallback; node; node = node->mNext) {
-    sizes.mCallbacks += mallocSizeOf(node);
-    sizes.mCallbacks += mallocSizeOf(node->mDomain.get());
+    node->AddSizeOfIncludingThis(mallocSizeOf, sizes);
   }
 
   MOZ_COLLECT_REPORT("explicit/preferences/hash-table",
                      KIND_HEAP,
                      UNITS_BYTES,
                      sizes.mHashTable,
                      "Memory used by libpref's hash table.");
 
@@ -2733,22 +2740,28 @@ PreferenceServiceReporter::CollectReport
                      "Memory used by libpref's root branches.");
 
   MOZ_COLLECT_REPORT("explicit/preferences/pref-name-arena",
                      KIND_HEAP,
                      UNITS_BYTES,
                      sizes.mPrefNameArena,
                      "Memory used by libpref's arena for pref names.");
 
-  MOZ_COLLECT_REPORT("explicit/preferences/callbacks",
+  MOZ_COLLECT_REPORT("explicit/preferences/callbacks/objects",
                      KIND_HEAP,
                      UNITS_BYTES,
-                     sizes.mCallbacks,
-                     "Memory used by libpref's callbacks list, including "
-                     "pref names and prefixes.");
+                     sizes.mCallbacksObjects,
+                     "Memory used by pref callback objects.");
+
+  MOZ_COLLECT_REPORT("explicit/preferences/callbacks/domains",
+                     KIND_HEAP,
+                     UNITS_BYTES,
+                     sizes.mCallbacksDomains,
+                     "Memory used by pref callback domains (pref names and "
+                     "prefixes).");
 
   MOZ_COLLECT_REPORT("explicit/preferences/misc",
                      KIND_HEAP,
                      UNITS_BYTES,
                      sizes.mMisc,
                      "Miscellaneous memory used by libpref.");
 
   nsPrefBranch* rootBranch =