copy from browser/config/mozconfigs/win32/mingw32
copy to browser/config/mozconfigs/win64/mingw32
--- a/browser/config/mozconfigs/win32/mingw32
+++ b/browser/config/mozconfigs/win64/mingw32
@@ -21,35 +21,38 @@ unset MAKECAB
 #    MOZ_AUTOMATION_ flags
 #  build/mozconfig.rust
 #    TOOLTOOL_DIR
 #    RUSTC
 #    CARGO
 . "$topsrcdir/browser/config/mozconfigs/common"
 
 # MinGW Stuff
-ac_add_options --target=i686-w64-mingw32
-ac_add_options --with-toolchain-prefix=i686-w64-mingw32-
+ac_add_options --target=x86_64-w64-mingw32
+ac_add_options --with-toolchain-prefix=x86_64-w64-mingw32-
 
 # GCC compiling for Windows exposes a lot of warnings. We are tracking them in Bug 1394433
 ac_add_options --disable-warnings-as-errors
 
 # Temporary config settings until we get these working on mingw
 ac_add_options --disable-accessibility # https://sourceforge.net/p/mingw-w64/bugs/648/
 
 # Long story
 ac_add_options --disable-stylo # Bug 1390583
+ac_add_options --disable-jemalloc # Bug 1466192
 
 # These aren't supported on mingw at this time
 ac_add_options --disable-webrtc # Bug 1393901
 ac_add_options --disable-maintenance-service
 
 # Find our toolchain
-CC="$TOOLTOOL_DIR/mingw32/bin/i686-w64-mingw32-gcc"
-CXX="$TOOLTOOL_DIR/mingw32/bin/i686-w64-mingw32-g++"
+CC="$TOOLTOOL_DIR/mingw32/bin/x86_64-w64-mingw32-gcc"
+CXX="$TOOLTOOL_DIR/mingw32/bin/x86_64-w64-mingw32-g++"
+
+CXXFLAGS="$CXXFLAGS -DWIDL_EXPLICIT_AGGREGATE_RETURNS"
 
 # We want to make sure we use binutils and other binaries in the tooltool
 # package.
 mk_add_options "export PATH=$TOOLTOOL_DIR/mingw32/bin:$TOOLTOOL_DIR/wine/bin:$TOOLTOOL_DIR/upx/bin:$TOOLTOOL_DIR/fxc2/bin:$PATH"
 
 LD_LIBRARY_PATH=${LD_LIBRARY_PATH:+$LD_LIBRARY_PATH:}$TOOLTOOL_DIR/mingw32/lib64
 mk_add_options "export LD_LIBRARY_PATH=$LD_LIBRARY_PATH"
 

copy from browser/config/mozconfigs/win32/mingw32-debug
copy to browser/config/mozconfigs/win64/mingw32-debug
--- a/browser/config/mozconfigs/win32/mingw32-debug
+++ b/browser/config/mozconfigs/win64/mingw32-debug
@@ -1,4 +1,4 @@
-. "$topsrcdir/browser/config/mozconfigs/win32/mingw32"
+. "$topsrcdir/browser/config/mozconfigs/win64/mingw32"
 
 ac_add_options --enable-debug
 ac_add_options --disable-optimize
\ No newline at end of file

--- a/taskcluster/ci/build/windows.yml
+++ b/taskcluster/ci/build/windows.yml
@@ -648,17 +648,17 @@ win32-mingw32/opt:
         platform: windows-mingw32/all
         symbol: WM32(Bo)
         tier: 2
     worker-type: aws-provisioner-v1/gecko-{level}-b-linux
     worker:
         docker-image: {in-tree: mingw32-build}
         max-run-time: 7200
         env:
-            PERFHERDER_EXTRA_OPTIONS: "opt"
+            PERFHERDER_EXTRA_OPTIONS: "opt 32"
     run:
         using: mozharness
         actions: [build]
         script: mozharness/scripts/fx_desktop_build.py
         config:
             - builds/releng_base_firefox.py
             - builds/releng_base_windows_32_mingw_builds.py
         need-xvfb: false
@@ -680,17 +680,17 @@ win32-mingw32/debug:
         platform: windows-mingw32/all
         symbol: WM32(Bd)
         tier: 2
     worker-type: aws-provisioner-v1/gecko-{level}-b-linux
     worker:
         docker-image: {in-tree: mingw32-build}
         max-run-time: 7200
         env:
-            PERFHERDER_EXTRA_OPTIONS: "debug"
+            PERFHERDER_EXTRA_OPTIONS: "debug 32"
     run:
         using: mozharness
         actions: [build]
         script: mozharness/scripts/fx_desktop_build.py
         config:
             - builds/releng_base_firefox.py
             - builds/releng_base_windows_32_mingw_builds.py
             - builds/releng_sub_windows_configs/32_mingw_debug.py
@@ -698,8 +698,73 @@ win32-mingw32/debug:
     toolchains:
         - mingw32-rust
         - linux64-upx
         - linux64-wine
         - linux64-sccache
         - linux64-mingw32-gcc
         - linux64-mingw32-nsis
         - linux64-mingw32-fxc2
+
+win64-mingw32/opt:
+    description: "Win64 MinGW Opt"
+    index:
+        product: firefox
+        job-name: win64-mingw32-opt
+    treeherder:
+        platform: windows-mingw32/all
+        symbol: WM64(Bo)
+        tier: 2
+    worker-type: aws-provisioner-v1/gecko-{level}-b-linux
+    worker:
+        docker-image: {in-tree: mingw32-build}
+        max-run-time: 7200
+        env:
+            PERFHERDER_EXTRA_OPTIONS: "opt 64"
+    run:
+        using: mozharness
+        actions: [build]
+        script: mozharness/scripts/fx_desktop_build.py
+        config:
+            - builds/releng_base_firefox.py
+            - builds/releng_base_windows_64_mingw_builds.py
+        need-xvfb: false
+    toolchains:
+        - mingw32-rust
+        - linux64-upx
+        - linux64-wine
+        - linux64-sccache
+        - linux64-mingw32-gcc
+        - linux64-mingw32-nsis
+        - linux64-mingw32-fxc2
+
+win64-mingw32/debug:
+    description: "Win64 MinGW Debug"
+    index:
+        product: firefox
+        job-name: win64-mingw32-debug
+    treeherder:
+        platform: windows-mingw32/all
+        symbol: WM64(Bd)
+        tier: 2
+    worker-type: aws-provisioner-v1/gecko-{level}-b-linux
+    worker:
+        docker-image: {in-tree: mingw32-build}
+        max-run-time: 72000
+        env:
+            PERFHERDER_EXTRA_OPTIONS: "debug 64"
+    run:
+        using: mozharness
+        actions: [build]
+        script: mozharness/scripts/fx_desktop_build.py
+        config:
+            - builds/releng_base_firefox.py
+            - builds/releng_base_windows_64_mingw_builds.py
+            - builds/releng_sub_windows_configs/64_mingw_debug.py
+        need-xvfb: false
+    toolchains:
+        - mingw32-rust
+        - linux64-upx
+        - linux64-wine
+        - linux64-sccache
+        - linux64-mingw32-gcc
+        - linux64-mingw32-nsis
+        - linux64-mingw32-fxc2

copy from testing/mozharness/configs/builds/releng_base_windows_32_mingw_builds.py
copy to testing/mozharness/configs/builds/releng_base_windows_64_mingw_builds.py
--- a/testing/mozharness/configs/builds/releng_base_windows_32_mingw_builds.py
+++ b/testing/mozharness/configs/builds/releng_base_windows_64_mingw_builds.py
@@ -27,20 +27,20 @@ config = {
          'min_scm_level': 2, 'default': 'try-build-has-no-secrets'},
     ],
     'vcs_share_base': '/builds/hg-shared',
     'enable_count_ctors': False,
     #########################################################################
 
 
     #########################################################################
-    ###### 32 bit specific ######
+    ###### 64 bit specific ######
     'base_name': 'WINNT_5.2_MINGW_%(branch)s',
-    'platform': 'win32-mingw32',
-    'stage_platform': 'win32-mingw32',
+    'platform': 'win64-mingw32',
+    'stage_platform': 'win64-mingw32',
     'env': {
         'MOZBUILD_STATE_PATH': os.path.join(os.getcwd(), '.mozbuild'),
         'DISPLAY': ':2',
         'HG_SHARE_BASE_DIR': '/builds/hg-shared',
         'MOZ_OBJDIR': '%(abs_obj_dir)s',
         'TINDERBOX_OUTPUT': '1',
         'TOOLTOOL_CACHE': '/builds/worker/tooltool-cache',
         'TOOLTOOL_HOME': '/builds',
@@ -48,12 +48,12 @@ config = {
         'LC_ALL': 'C',
         # 32 bit specific
         'PATH': '/usr/local/bin:/bin:/usr/bin',
     },
     "check_test_env": {
         'MINIDUMP_STACKWALK': '%(abs_tools_dir)s/breakpad/linux/minidump_stackwalk',
         'MINIDUMP_SAVE_PATH': '%(base_work_dir)s/minidumps',
     },
-    'mozconfig_platform': 'win32',
+    'mozconfig_platform': 'win64',
     'mozconfig_variant': 'mingw32',
     #########################################################################
 }

copy from testing/mozharness/configs/builds/releng_sub_windows_configs/32_mingw_debug.py
copy to testing/mozharness/configs/builds/releng_sub_windows_configs/64_mingw_debug.py