--- a/mfbt/Endian.h
+++ b/mfbt/Endian.h
@@ -117,24 +117,24 @@
  * here because they're not present on all platforms.  Instead we have
  * this big conditional that ideally will catch all the interesting
  * cases.
  */
 #elif defined(__sparc) || defined(__sparc__) || \
       defined(_POWER) || defined(__powerpc__) || \
       defined(__ppc__) || defined(__hppa) || \
       defined(_MIPSEB) || defined(__ARMEB__) || \
-      defined(__s390__) || \
+      defined(__s390__) || defined(__AARCH64EB__) \
       (defined(__sh__) && defined(__LITTLE_ENDIAN__)) || \
       (defined(__ia64) && defined(__BIG_ENDIAN__))
 #  define MOZ_BIG_ENDIAN 1
 #elif defined(__i386) || defined(__i386__) || \
       defined(__x86_64) || defined(__x86_64__) || \
       defined(_MIPSEL) || defined(__ARMEL__) || \
-      defined(__alpha__) || \
+      defined(__alpha__) || defined(__AARCH64EL__) \
       (defined(__sh__) && defined(__BIG_ENDIAN__)) || \
       (defined(__ia64) && !defined(__BIG_ENDIAN__))
 #  define MOZ_LITTLE_ENDIAN 1
 #endif
 
 #if MOZ_BIG_ENDIAN
 #  define MOZ_LITTLE_ENDIAN 0
 #elif MOZ_LITTLE_ENDIAN

--- a/mfbt/tests/TestPoisonArea.cpp
+++ b/mfbt/tests/TestPoisonArea.cpp
@@ -156,16 +156,19 @@
 /* On mipsel, jr ra needs to be followed by a nop.
    0x03e00008 as a 64 bits integer just does that */
 #define RETURN_INSTR_TYPE uint64_t
 #endif
 
 #elif defined __s390__
 #define RETURN_INSTR 0x07fe0000 /* br %r14 */
 
+#elif defined __aarch64__
+#define RETURN_INSTR 0xd65f03c0 /* ret */
+
 #elif defined __ia64
 struct ia64_instr { uint32_t i[4]; };
 static const ia64_instr _return_instr =
   {{ 0x00000011, 0x00000001, 0x80000200, 0x00840008 }}; /* br.ret.sptk.many b0 */
 
 #define RETURN_INSTR _return_instr
 #define RETURN_INSTR_TYPE ia64_instr