Related
https://bugzilla.mozilla.org/show_bug.cgi?id=1543659
https://bugzilla.mozilla.org/show_bug.cgi?id=1543843
https://hg.mozilla.org/mozilla-central/rev/bc11fcee5741
https://bugzilla.mozilla.org/show_bug.cgi?id=1488552
https://salsa.debian.org/gnome-team/mozjs60/blob/debian/master/debian/patches/jsproperty-endian.patch

diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 0f11787..5042af2 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -1581,7 +1581,12 @@ struct JSPropertySpec {
     uintptr_t type;
     union {
       const char* string;
-      int32_t int32;
+      struct {
+#if MOZ_BIG_ENDIAN && JS_BITS_PER_WORD == 64
+        uint32_t    padding;
+#endif
+        int32_t     int32;
+      };
     };
   };
 
Bug 1488552 - Ensure proper running on 64-bit and 32-bit BE platforms.

diff --git a/js/src/gc/RelocationOverlay.h b/js/src/gc/RelocationOverlay.h
--- a/js/src/gc/RelocationOverlay.h
+++ b/js/src/gc/RelocationOverlay.h
@@ -29,23 +29,34 @@ struct Cell;
  * This structure overlays a Cell that has been moved and provides a way to find
  * its new location. It's used during generational and compacting GC.
  */
 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_;
 
   /* A list entry to track all relocated things. */
   RelocationOverlay* next_;
 
  public:
diff --git a/js/src/vm/StringType.h b/js/src/vm/StringType.h
--- a/js/src/vm/StringType.h
+++ b/js/src/vm/StringType.h
@@ -2,16 +2,17 @@
  * vim: set ts=8 sts=4 et sw=4 tw=99:
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef vm_StringType_h
 #define vm_StringType_h
 
+#include "mozilla/EndianUtils.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/PodOperations.h"
 #include "mozilla/Range.h"
 
 #include "jsapi.h"
 #include "jsfriendapi.h"
 
 #include "builtin/String.h"
@@ -168,8 +168,20 @@ 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;
--- thunderbird-60.2.1/js/src/gc/Marking-inl.h.old	2018-10-01 14:51:12.000000000 +0000
+++ thunderbird-60.2.1/js/src/gc/Marking-inl.h	2018-10-12 19:08:28.260000000 +0000
@@ -92,13 +92,29 @@
   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");
   magic_ = Relocated;
--- thunderbird-60.2.1/js/src/jsfriendapi.h.old	2018-10-01 14:51:13.000000000 +0000
+++ thunderbird-60.2.1/js/src/jsfriendapi.h	2018-10-12 19:12:06.190000000 +0000
@@ -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"
@@ -640,8 +641,15 @@
   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;