diff --git a/js/src/gc/Marking-inl.h b/js/src/gc/Marking-inl.h
index 6d2a4c7..c773c21 100644
--- a/js/src/gc/Marking-inl.h
+++ b/js/src/gc/Marking-inl.h
@@ -82,12 +82,28 @@ inline void RelocationOverlay::forwardTo(Cell* cell) {
   MOZ_ASSERT(!isForwarded());
   // The location of magic_ is important because it must never be valid to see
   // the value Relocated there in a GC thing that has not been moved.
+#if MOZ_LITTLE_ENDIAN || JS_BITS_PER_WORD == 32
+  // On 32-bit, the magic_ aliases with whatever comes after the first
+  // pointer; on little-endian 64-bit, the magic_ aliases with the
+  // 32 most significant bits of the pointer, which are the second half.
   static_assert(offsetof(RelocationOverlay, magic_) ==
                     offsetof(JSObject, group_) + sizeof(uint32_t),
                 "RelocationOverlay::magic_ is in the wrong location");
   static_assert(offsetof(RelocationOverlay, magic_) ==
                     offsetof(js::Shape, base_) + sizeof(uint32_t),
                 "RelocationOverlay::magic_ is in the wrong location");
+#elif JS_BITS_PER_WORD == 64
+  // On big-endian 64-bit, the magic_ aliases with the 32 most
+  // significant bits of the pointer, but now that's the first half.
+  static_assert(offsetof(RelocationOverlay, magic_) ==
+                    offsetof(JSObject, group_),
+                "RelocationOverlay::magic_ is in the wrong location");
+  static_assert(offsetof(RelocationOverlay, magic_) ==
+                    offsetof(js::Shape, base_),
+                "RelocationOverlay::magic_ is in the wrong location");
+#else
+#  error "Unknown endianness or word size"
+#endif
   static_assert(
       offsetof(RelocationOverlay, magic_) == offsetof(JSString, d.u1.length),
       "RelocationOverlay::magic_ is in the wrong location");
diff --git a/js/src/gc/RelocationOverlay.h b/js/src/gc/RelocationOverlay.h
index a568843..399a541 100644
--- a/js/src/gc/RelocationOverlay.h
+++ b/js/src/gc/RelocationOverlay.h
@@ -33,14 +33,25 @@ class RelocationOverlay {
   /* See comment in js/public/HeapAPI.h. */
   static const uint32_t Relocated = js::gc::Relocated;
 
+#if MOZ_LITTLE_ENDIAN || JS_BITS_PER_WORD == 32
   /*
-   * Keep the low 32 bits untouched. Use them to distinguish strings from
+   * Keep the first 32 bits untouched. Use them to distinguish strings from
    * objects in the nursery.
    */
   uint32_t preserve_;
 
   /* Set to Relocated when moved. */
   uint32_t magic_;
+#elif JS_BITS_PER_WORD == 64
+  /*
+   * On big-endian, we need to reorder to keep preserve_ lined up with the
+   * low 32 bits of the aligned group_ pointer in JSObject.
+   */
+  uint32_t magic_;
+  uint32_t preserve_;
+#else
+#  error "Unknown endianness or word size"
+#endif
 
   /* The location |this| was moved to. */
   Cell* newLocation_;
diff --git a/js/src/js-config.h.in b/js/src/js-config.h.in
index f2ce93e..4034315 100644
--- a/js/src/js-config.h.in
+++ b/js/src/js-config.h.in
@@ -38,6 +38,13 @@
 /* Define to 1 if SpiderMonkey should include ctypes support.  */
 #undef JS_HAS_CTYPES
 
+/* Define to 1 if SpiderMonkey should include trace logging support. */
+#undef JS_TRACE_LOGGING
+
+/* Define to 1 if SpiderMonkey should include a breakpoint function for
+ * artificial OOMs. */
+#undef JS_OOM_BREAKPOINT
+
 /* Define to 1 if SpiderMonkey should support the ability to perform
    entirely too much GC.  */
 #undef JS_GC_ZEAL
@@ -48,12 +55,25 @@
 /* Define to 1 to perform extra assertions and heap poisoning. */
 #undef JS_CRASH_DIAGNOSTICS
 
+/* Define to 1 if SpiderMonkey is compiled with support for private values at
+ * odd-numbered memory addresses. */
+#undef JS_UNALIGNED_PRIVATE_VALUES
+
 /* Define to 1 if SpiderMonkey is in NUNBOX32 mode. */
 #undef JS_NUNBOX32
 
 /* Define to 1 if SpiderMonkey is in PUNBOX64 mode. */
 #undef JS_PUNBOX64
 
+/* Define exactly one of these to 1 to indicate JIT code generation mode. */
+#undef JS_CODEGEN_NONE
+#undef JS_CODEGEN_ARM
+#undef JS_CODEGEN_ARM64
+#undef JS_CODEGEN_MIPS32
+#undef JS_CODEGEN_MIPS64
+#undef JS_CODEGEN_X86
+#undef JS_CODEGEN_X64
+
 /* MOZILLA JSAPI version number components */
 #undef MOZJS_MAJOR_VERSION
 #undef MOZJS_MINOR_VERSION
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index d7b3d90..0ada71e 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -115,6 +115,14 @@ using mozilla::Some;
 #define JS_ADDRESSOF_VA_LIST(ap) (&(ap))
 #endif
 
+// See preprocessor definition of JS_BITS_PER_WORD in jstypes.h; make sure
+// JS_64BIT (used internally) agrees with it
+#ifdef JS_64BIT
+static_assert(JS_BITS_PER_WORD == 64, "values must be in sync");
+#else
+static_assert(JS_BITS_PER_WORD == 32, "values must be in sync");
+#endif
+
 JS_PUBLIC_API bool JS::CallArgs::requireAtLeast(JSContext* cx,
                                                 const char* fnname,
                                                 unsigned required) const {
diff --git a/js/src/jsfriendapi.h b/js/src/jsfriendapi.h
index 4b8d18a..70ce0a1 100644
--- a/js/src/jsfriendapi.h
+++ b/js/src/jsfriendapi.h
@@ -9,6 +9,7 @@
 
 #include "mozilla/Atomics.h"
 #include "mozilla/Casting.h"
+#include "mozilla/EndianUtils.h"
 #include "mozilla/Maybe.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/UniquePtr.h"
@@ -609,8 +610,15 @@ struct String {
   static const uint32_t LATIN1_CHARS_BIT = JS_BIT(6);
   static const uint32_t EXTERNAL_FLAGS = LINEAR_BIT | NON_ATOM_BIT | JS_BIT(5);
   static const uint32_t TYPE_FLAGS_MASK = JS_BIT(6) - 1;
+#if MOZ_LITTLE_ENDIAN || JS_BITS_PER_WORD == 32
   uint32_t flags;
   uint32_t length;
+#elif JS_BITS_PER_WORD == 64
+  uint32_t length;
+  uint32_t flags;
+#else
+#  error "Unknown endianness or word size"
+#endif
   union {
     const JS::Latin1Char* nonInlineCharsLatin1;
     const char16_t* nonInlineCharsTwoByte;
diff --git a/js/src/jstypes.h b/js/src/jstypes.h
index ace4f0f..ac7b275 100644
--- a/js/src/jstypes.h
+++ b/js/src/jstypes.h
@@ -141,12 +141,17 @@
 #define JS_BITS_PER_BYTE 8
 #define JS_BITS_PER_BYTE_LOG2 3
 
-#if defined(JS_64BIT)
+#if (defined(__SIZEOF_POINTER__) && __SIZEOF_POINTER__ == 8) || \
+    (defined(UINTPTR_MAX) && UINTPTR_MAX == 0xFFFFFFFFFFFFFFFFu)
 #define JS_BITS_PER_WORD 64
 #else
 #define JS_BITS_PER_WORD 32
 #endif
 
+static_assert(sizeof(void*) == 8 ? JS_BITS_PER_WORD == 64
+                                 : JS_BITS_PER_WORD == 32,
+              "preprocessor and compiler must agree");
+
 /***********************************************************************
 ** MACROS:      JS_FUNC_TO_DATA_PTR
 **              JS_DATA_TO_FUNC_PTR
diff --git a/js/src/vm/StringType.h b/js/src/vm/StringType.h
index cde3427..c3400db 100644
--- a/js/src/vm/StringType.h
+++ b/js/src/vm/StringType.h
@@ -7,6 +7,7 @@
 #ifndef vm_StringType_h
 #define vm_StringType_h
 
+#include "mozilla/EndianUtils.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/PodOperations.h"
 #include "mozilla/Range.h"
@@ -168,8 +169,20 @@ class JSString : public js::gc::Cell {
   struct Data {
     union {
       struct {
+#if MOZ_LITTLE_ENDIAN || JS_BITS_PER_WORD == 32
         uint32_t flags;  /* JSString */
         uint32_t length; /* JSString */
+#elif JS_BITS_PER_WORD == 64
+        /*
+         * On big-endian, we need to reorder to keep flags lined up
+         * with the low 32 bits of the aligned group_ pointer in
+         * JSObject.
+         */
+        uint32_t length; /* JSString */
+        uint32_t flags;  /* JSString */
+#else
+#  error "Unknown endianness or word size"
+#endif
       };
       uintptr_t flattenData; /* JSRope (temporary while flattening) */
     } u1;