diff -Nurp a/build/moz.configure/init.configure b/build/moz.configure/init.configure
--- a/build/moz.configure/init.configure	2020-08-18 02:26:21.000000000 +0800
+++ b/build/moz.configure/init.configure	2021-09-12 12:10:40.998763175 +0800
@@ -734,6 +734,9 @@ def split_triplet(triplet, allow_unknown
     elif cpu == 'sh4':
         canonical_cpu = 'sh4'
         endianness = 'little'
+    elif cpu == 'loongarch64':
+        canonical_cpu = 'loongarch64'
+        endianness = 'little'
     elif allow_unknown:
         canonical_cpu = cpu
         endianness = 'unknown'
diff -Nurp a/js/src/jit/AtomicOperations.h b/js/src/jit/AtomicOperations.h
--- a/js/src/jit/AtomicOperations.h	2021-09-12 02:21:25.181831470 +0800
+++ b/js/src/jit/AtomicOperations.h	2021-09-12 12:13:47.990155312 +0800
@@ -382,6 +382,9 @@ inline bool AtomicOperations::isLockfree
 #  else
 #    include "jit/shared/AtomicOperations-feeling-lucky.h"
 #  endif
+#elif defined(__loongarch64)
+// This is only here to avoid merge conflict with spidermonkey-68.0-add-riscv-support.patch.
+#  include "jit/shared/AtomicOperations-feeling-lucky.h"
 #elif defined(__mips__)
 #  if defined(__clang__) || defined(__GNUC__)
 #    include "jit/mips-shared/AtomicOperations-mips-shared.h"
diff -Nurp a/js/src/jit/shared/AtomicOperations-feeling-lucky-gcc.h b/js/src/jit/shared/AtomicOperations-feeling-lucky-gcc.h
--- a/js/src/jit/shared/AtomicOperations-feeling-lucky-gcc.h	2020-08-18 02:26:24.000000000 +0800
+++ b/js/src/jit/shared/AtomicOperations-feeling-lucky-gcc.h	2021-09-12 12:15:04.714310338 +0800
@@ -57,6 +57,11 @@
 #  define HAS_64BIT_LOCKFREE
 #endif
 
+#ifdef __loongarch64
+#  define HAS_64BIT_ATOMICS
+#  define HAS_64BIT_LOCKFREE
+#endif
+
 #if defined(__ppc64__) || defined(__PPC64__) || defined(__ppc64le__) || \
     defined(__PPC64LE__)
 #  define HAS_64BIT_ATOMICS
diff -Nurp a/mfbt/tests/TestPoisonArea.cpp b/mfbt/tests/TestPoisonArea.cpp
--- a/mfbt/tests/TestPoisonArea.cpp	2020-08-18 02:26:28.000000000 +0800
+++ b/mfbt/tests/TestPoisonArea.cpp	2021-09-12 12:16:49.447517636 +0800
@@ -127,6 +127,9 @@
 #elif defined __arm__ || defined _M_ARM
 #  define RETURN_INSTR 0xE12FFF1E /* bx lr */
 
+#elif defined __loongarch__
+#  define RETURN_INSTR 0x4c000020 /* jirl zero, ra, 0 */
+
 // PPC has its own style of CPU-id #defines.  There is no Windows for
 // PPC as far as I know, so no _M_ variant.
 #elif defined _ARCH_PPC || defined _ARCH_PWR || defined _ARCH_PWR2
diff -Nurp a/python/mozbuild/mozbuild/configure/constants.py b/python/mozbuild/mozbuild/configure/constants.py
--- a/python/mozbuild/mozbuild/configure/constants.py	2020-08-18 02:26:28.000000000 +0800
+++ b/python/mozbuild/mozbuild/configure/constants.py	2021-09-12 12:19:00.762771788 +0800
@@ -46,6 +46,7 @@ CPU_bitness = {
     'arm': 32,
     'hppa': 32,
     'ia64': 64,
+    'loongarch64': 64,
     'mips32': 32,
     'mips64': 64,
     'ppc': 32,
@@ -78,6 +79,7 @@ CPU_preprocessor_checks = OrderedDict((
     ('arm', '__arm__ || _M_ARM'),
     ('aarch64', '__aarch64__ || _M_ARM64'),
     ('ia64', '__ia64__'),
+    ('loongarch64', '__loongarch64'),
     ('s390x', '__s390x__'),
     ('s390', '__s390__'),
     ('ppc64', '__powerpc64__'),
diff -Nurp a/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py b/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py
--- a/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py	2020-08-18 02:26:28.000000000 +0800
+++ b/python/mozbuild/mozbuild/test/configure/test_toolchain_configure.py	2021-09-12 12:20:13.765910816 +0800
@@ -1068,6 +1068,10 @@ class LinuxCrossCompileToolchainTest(Bas
         'ia64-unknown-linux-gnu': little_endian + {
             '__ia64__': 1,
         },
+        'loongarch64-unknown-linux-gnu': little_endian + {
+            '__loongarch__': 1,
+            '__loongarch64': 1,
+        },
         's390x-unknown-linux-gnu': big_endian + {
             '__s390x__': 1,
             '__s390__': 1,
diff -Nurp a/intl/icu/source/i18n/double-conversion-utils.h b/intl/icu/source/i18n/double-conversion-utils.h
--- a/intl/icu/source/i18n/double-conversion-utils.h	2021-04-08 00:10:27.000000000 +0000
+++ b/intl/icu/source/i18n/double-conversion-utils.h	2021-08-21 09:18:06.800791611 +0000
@@ -117,6 +117,7 @@ int main(int argc, char** argv) {
 #if defined(_M_X64) || defined(__x86_64__) || \
     defined(__ARMEL__) || defined(__avr32__) || defined(_M_ARM) || defined(_M_ARM64) || \
     defined(__hppa__) || defined(__ia64__) || \
+    defined(__loongarch__) || \
     defined(__mips__) || \
     defined(__nios2__) || defined(__ghs) || \
     defined(__powerpc__) || defined(__ppc__) || defined(__ppc64__) || \
diff -Nurp a/mfbt/double-conversion/double-conversion/utils.h b/mfbt/double-conversion/double-conversion/utils.h
--- a/mfbt/double-conversion/double-conversion/utils.h	2021-04-08 00:10:27.000000000 +0000
+++ b/mfbt/double-conversion/double-conversion/utils.h	2021-08-21 09:18:06.800791611 +0000
@@ -117,6 +117,7 @@ int main(int argc, char** argv) {
 #if defined(_M_X64) || defined(__x86_64__) || \
     defined(__ARMEL__) || defined(__avr32__) || defined(_M_ARM) || defined(_M_ARM64) || \
     defined(__hppa__) || defined(__ia64__) || \
+    defined(__loongarch__) || \
     defined(__mips__) || \
     defined(__nios2__) || defined(__ghs) || \
     defined(__powerpc__) || defined(__ppc__) || defined(__ppc64__) || \