diff --git a/xpcom/typelib/xpt/public/xpt_arena.h b/xpcom/typelib/xpt/public/xpt_arena.h
index c4f208806a80..1192aada7f3c 100644
--- a/xpcom/typelib/xpt/public/xpt_arena.h
+++ b/xpcom/typelib/xpt/public/xpt_arena.h
@@ -12,6 +12,7 @@
 
 #include "prtypes.h"
 #include <stdlib.h>
+#include "mozilla/MemoryReporting.h"
 #include "mozilla/StandardInteger.h"
 
 
@@ -53,11 +54,8 @@ XPT_NotifyDoneLoading(XPTArena *arena);
 XPT_PUBLIC_API(void)
 XPT_ArenaFree(XPTArena *arena, void* block);
 
-/* A synonym of |nsMallocSizeOfFun|, because we don't #include nscore.h. */
-typedef size_t(*xptMallocSizeOfFun)(const void *p);
-
 XPT_PUBLIC_API(size_t)
-XPT_SizeOfArena(XPTArena *arena, xptMallocSizeOfFun mallocSizeOf);
+XPT_SizeOfArena(XPTArena *arena, MozMallocSizeOf mallocSizeOf);
 
 /* --------------------------------------------------------- */
 
diff --git a/xpcom/typelib/xpt/src/xpt_arena.c b/xpcom/typelib/xpt/src/xpt_arena.c
index 8efaa08b5609..c4bb161c5d4b 100644
--- a/xpcom/typelib/xpt/src/xpt_arena.c
+++ b/xpcom/typelib/xpt/src/xpt_arena.c
@@ -12,6 +12,7 @@
 */
 
 #include "xpt_arena.h"
+#include "mozilla/MemoryReporting.h"
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -306,7 +307,7 @@ XPT_AssertFailed(const char *s, const char *file, uint32_t lineno)
 #endif
 
 XPT_PUBLIC_API(size_t)
-XPT_SizeOfArena(XPTArena *arena, xptMallocSizeOfFun mallocSizeOf)
+XPT_SizeOfArena(XPTArena *arena, MozMallocSizeOf mallocSizeOf)
 {
     size_t n = mallocSizeOf(arena);