diff --git a/mfbt/TypedEnum.h b/mfbt/TypedEnum.h
index 22f59decf2a..56ada45d609 100644
--- a/mfbt/TypedEnum.h
+++ b/mfbt/TypedEnum.h
@@ -10,6 +10,8 @@
 
 #include "mozilla/Attributes.h"
 
+#if defined(__cplusplus)
+
 #if defined(__clang__)
    /*
     * Per Clang documentation, "Note that marketing version numbers should not
@@ -211,4 +213,6 @@
      inline int& operator>>=(int&, const Name::Enum&) MOZ_DELETE;
 #endif
 
+#endif /* __cplusplus */
+
 #endif  /* mozilla_TypedEnum_h_ */
diff --git a/xpcom/base/Makefile.in b/xpcom/base/Makefile.in
index 51de8df8758..359ac08b8a6 100644
--- a/xpcom/base/Makefile.in
+++ b/xpcom/base/Makefile.in
@@ -54,6 +54,10 @@ ifeq (cocoa,$(MOZ_WIDGET_TOOLKIT))
 CPPSRCS		+= nsMacUtilsImpl.cpp
 endif
 
+CSRCS = \
+		nsErrorAssertsC.c \
+		$(NULL)
+
 EXPORTS		= \
 		nsAgg.h \
 		nsAutoRef.h \
diff --git a/xpcom/base/nsError.h b/xpcom/base/nsError.h
index b9310af9c7d..fa89a810243 100644
--- a/xpcom/base/nsError.h
+++ b/xpcom/base/nsError.h
@@ -7,12 +7,9 @@
 #define nsError_h__
 
 #include "mozilla/Likely.h"
+#include "mozilla/StandardInteger.h"
 #include "mozilla/TypedEnum.h"
 
-#ifndef nscore_h___
-#include "nscore.h"  /* needed for nsresult */
-#endif
-
 /*
  * To add error code to your module, you need to do the following:
  *
@@ -120,16 +117,8 @@
  * either can be converted to the other, so it's ambiguous.  So we have to fall
  * back to a regular enum.
  */
-#if defined(__cplusplus)
 #if defined(MOZ_HAVE_CXX11_STRONG_ENUMS)
   typedef enum class tag_nsresult : uint32_t
-#elif defined(MOZ_HAVE_CXX11_ENUM_TYPE)
-  /* Need underlying type for workaround of Microsoft compiler (Bug 794734) */
-  typedef enum tag_nsresult : uint32_t
-#else
-  /* no strong enums */
-  typedef enum tag_nsresult
-#endif
   {
     #undef ERROR
     #define ERROR(key, val) key = val
@@ -137,18 +126,44 @@
     #undef ERROR
   } nsresult;
 
-#if defined(MOZ_HAVE_CXX11_STRONG_ENUMS)
-  /* We're using enum classes, so we need #define's to put the constants in
-   * global scope for compatibility with old code. */
+  /*
+   * enum classes don't place their initializers in the global scope, so we need
+   * #define's for compatibility with old code.
+   */
   #include "ErrorListCxxDefines.h"
-#endif
-#else /* defined(__cplusplus) */
-  /* C enum can't have a value outside the range of 'int'.
-   * C const can't initialize with an expression involving other variables
-   * even if it is const. So we have to fall back to bad old #defines. */
+#elif defined(MOZ_HAVE_CXX11_ENUM_TYPE)
+  typedef enum tag_nsresult : uint32_t
+  {
+    #undef ERROR
+    #define ERROR(key, val) key = val
+    #include "ErrorList.h"
+    #undef ERROR
+  } nsresult;
+#elif defined(__cplusplus)
+  /*
+   * We're C++ in an old compiler lacking enum classes *and* typed enums (likely
+   * gcc < 4.5.1 as clang/MSVC have long supported one or both), or compiler
+   * support is unknown.  Yet nsresult must have unsigned 32-bit representation.
+   * So just make it a typedef, and implement the constants with global consts.
+   */
+  typedef uint32_t nsresult;
+
+  const nsresult
+  #undef ERROR
+  #define ERROR(key, val) key = val
+  #include "ErrorList.h"
+  #undef ERROR
+    ;
+#else
+  /*
+   * C doesn't have any way to fix the type underlying an enum, and enum
+   * initializers can't have values outside the range of 'int'.  So typedef
+   * nsresult to the correct unsigned type, and fall back to using #defines for
+   * all error constants.
+   */
   typedef uint32_t nsresult;
   #include "ErrorListCDefines.h"
-#endif /* defined(__cplusplus) */
+#endif
 
 #undef SUCCESS_OR_FAILURE
 #undef SUCCESS
diff --git a/xpcom/base/nsErrorAsserts.cpp b/xpcom/base/nsErrorAsserts.cpp
index 1a7b38a6944..b987a7960e8 100644
--- a/xpcom/base/nsErrorAsserts.cpp
+++ b/xpcom/base/nsErrorAsserts.cpp
@@ -1,7 +1,10 @@
-#include "nsError.h"
 #include "mozilla/Assertions.h"
 
+#include "nsError.h"
+
 // No reason to pull in Assertions.h for every single file that includes
 // nsError.h, so let's put this in its own .cpp file instead of in the .h.
+MOZ_STATIC_ASSERT(((nsresult)0) < ((nsresult)-1),
+                  "nsresult must be an unsigned type");
 MOZ_STATIC_ASSERT(sizeof(nsresult) == sizeof(uint32_t),
                   "nsresult must be 32 bits");
diff --git a/xpcom/base/nsErrorAssertsC.c b/xpcom/base/nsErrorAssertsC.c
new file mode 100644
index 00000000000..de73978f88b
--- /dev/null
+++ b/xpcom/base/nsErrorAssertsC.c
@@ -0,0 +1,12 @@
+#include "mozilla/Assertions.h"
+
+#include "nsError.h"
+
+/*
+ * No reason to pull in Assertions.h for every single file that includes
+ * nsError.h, so let's put this in its own .c file instead of in the .h.
+ */
+MOZ_STATIC_ASSERT(((nsresult)0) < ((nsresult)-1),
+                  "nsresult must be an unsigned type");
+MOZ_STATIC_ASSERT(sizeof(nsresult) == sizeof(uint32_t),
+                  "nsresult must be 32 bits");
diff --git a/xpcom/base/nscore.h b/xpcom/base/nscore.h
index 8ea5031c79b..e1e8f533d46 100644
--- a/xpcom/base/nscore.h
+++ b/xpcom/base/nscore.h
@@ -325,8 +325,6 @@ typedef unsigned long nsrefcnt;
 typedef uint32_t nsrefcnt;
 #endif
 
-#include "nsError.h"
-
 /* ------------------------------------------------------------------------ */
 /* Casting macros for hiding C++ features from older compilers */