diff --git a/memory/mozalloc/mozalloc.cpp b/memory/mozalloc/mozalloc.cpp
index 7f752899bbb..f4c3f96decc 100644
--- a/memory/mozalloc/mozalloc.cpp
+++ b/memory/mozalloc/mozalloc.cpp
@@ -48,6 +48,10 @@
 #include <stddef.h>             // for size_t
 #include <stdlib.h>             // for malloc, free
 
+#if defined(XP_WIN) || (defined(XP_OS2) && defined(__declspec))
+#  define MOZALLOC_EXPORT __declspec(dllexport)
+#endif
+
 // Make sure that "malloc" et al. resolve to their libc variants.
 #define MOZALLOC_DONT_DEFINE_MACRO_WRAPPERS
 #include "mozilla/mozalloc.h"
diff --git a/memory/mozalloc/mozalloc.h b/memory/mozalloc/mozalloc.h
index ea50220fc6e..cd42eb89cd7 100644
--- a/memory/mozalloc/mozalloc.h
+++ b/memory/mozalloc/mozalloc.h
@@ -54,8 +54,11 @@
  * I.e., we don't #include <stdlib.h> or <new> on purpose.
  */
 
-#if defined(XP_WIN) || (defined(XP_OS2) && defined(__declspec))
-#  define MOZALLOC_EXPORT __declspec(dllexport)
+#if defined(MOZALLOC_EXPORT)
+// do nothing: it's been defined to __declspec(dllexport) by
+// mozalloc*.cpp on platforms where that's required
+#elif defined(XP_WIN) || (defined(XP_OS2) && defined(__declspec))
+#  define MOZALLOC_EXPORT __declspec(dllimport)
 #elif defined(HAVE_VISIBILITY_ATTRIBUTE)
 /* Make sure symbols are still exported even if we're wrapped in a
  * |visibility push(hidden)| blanket. */
diff --git a/memory/mozalloc/mozalloc_oom.cpp b/memory/mozalloc/mozalloc_oom.cpp
index 84924a80a5a..8d4878abb20 100644
--- a/memory/mozalloc/mozalloc_oom.cpp
+++ b/memory/mozalloc/mozalloc_oom.cpp
@@ -46,6 +46,10 @@
 #  include <unistd.h>           // for _exit
 #endif
 
+#if defined(XP_WIN) || (defined(XP_OS2) && defined(__declspec))
+#  define MOZALLOC_EXPORT __declspec(dllexport)
+#endif
+
 #include "mozilla/mozalloc_oom.h"
 
 static int gDummyCounter;
diff --git a/memory/mozalloc/mozalloc_oom.h b/memory/mozalloc/mozalloc_oom.h
index 86b4fc95537..311169c0521 100644
--- a/memory/mozalloc/mozalloc_oom.h
+++ b/memory/mozalloc/mozalloc_oom.h
@@ -41,9 +41,11 @@
 #ifndef mozilla_mozalloc_oom_h
 #define mozilla_mozalloc_oom_h
 
-
-#if defined(XP_WIN) || (defined(XP_OS2) && defined(__declspec))
-#  define MOZALLOC_EXPORT __declspec(dllexport)
+#if defined(MOZALLOC_EXPORT)
+// do nothing: it's been defined to __declspec(dllexport) by
+// mozalloc*.cpp on platforms where that's required
+#elif defined(XP_WIN) || (defined(XP_OS2) && defined(__declspec))
+#  define MOZALLOC_EXPORT __declspec(dllimport)
 #elif defined(HAVE_VISIBILITY_ATTRIBUTE)
 /* Make sure symbols are still exported even if we're wrapped in a
  * |visibility push(hidden)| blanket. */
diff --git a/xpcom/base/nscore.h b/xpcom/base/nscore.h
index bede7272ede..e0227cb0b50 100644
--- a/xpcom/base/nscore.h
+++ b/xpcom/base/nscore.h
@@ -50,7 +50,10 @@
 #  if defined(__cplusplus)
 #    include NEW_H              /* to give mozalloc std::bad_alloc */
 #  endif
-#  include <stdlib.h>         /* to give mozalloc malloc/free decls */
+/* include these to avoid the macro wrappers causing these headers to
+ * declare system functions with moz_ prefixes */
+#  include <stdlib.h>
+#  include <string.h>
 #  include "mozilla/mozalloc.h"
 #  include "mozilla/mozalloc_macro_wrappers.h"
 #endif