diff --git a/browser/config/mozconfigs/win64/cross b/browser/config/mozconfigs/win64/cross
new file mode 100644
index 000000000000..0bd2a2110070
--- /dev/null
+++ b/browser/config/mozconfigs/win64/cross
@@ -0,0 +1,37 @@
+APIKEYDIR=/builds
+
+. "$topsrcdir/build/mozconfig.win-common"
+. "$topsrcdir/browser/config/mozconfigs/win64/common-win64"
+. "$topsrcdir/browser/config/mozconfigs/win64/common-opt"
+
+ac_add_options --with-branding=browser/branding/nightly
+
+# Configure expects executables for check_prog, so set the relevant files
+# as executable on the first evaluation of the mozconfig where they exist.
+if [ -d "${VSPATH}" -a ! -x "${VSPATH}/VC/bin/Hostx64/x64/cl.exe" ]; then
+    find "${VSPATH}" -type f -name \*.exe -exec chmod +x {} \;
+fi
+export MAKENSISU="${MOZ_FETCHES_DIR}/nsis-3.01/makensis.exe"
+if [ -d "${MAKENSISU%/*}" -a ! -x "${MAKENSISU}" ]; then
+    chmod +x "${MAKENSISU}"
+fi
+export MAKECAB="${TOOLTOOL_DIR}/makecab.exe"
+if [ -f "${MAKECAB}" ]; then
+    chmod +x "${MAKECAB}"
+fi
+export UPX="${MOZ_FETCHES_DIR}/upx-3.95-win64/upx.exe"
+if [ -f "${UPX}" ]; then
+    chmod +x "${UPX}"
+fi
+CLANG_CL_EXE_DIR="${MOZ_FETCHES_DIR}/clang-cl/clang/bin"
+if [ -d "${CLANG_CL_EXE_DIR}" ]; then
+    chmod +x "${CLANG_CL_EXE_DIR}/clang-cl.exe"
+fi
+export WINE=${MOZ_FETCHES_DIR}/wine/bin/wine64
+export RC=rc.exe
+export MIDL=midl.exe
+mk_add_options "export PATH=${TOOLTOOL_DIR}/vs2017_15.8.4/VC/bin/Hostx64/x64:${MOZ_FETCHES_DIR}/nsis-3.01:${CLANG_CL_EXE_DIR}:${PATH}"
+
+unset VC_PATH
+
+. "$topsrcdir/build/mozconfig.common.override"
diff --git a/build/mozconfig.clang-cl b/build/mozconfig.clang-cl
index cfc7a2a8fc0c..cb32289abefb 100644
--- a/build/mozconfig.clang-cl
+++ b/build/mozconfig.clang-cl
@@ -13,7 +13,14 @@ export CXX=clang-cl
 export ENABLE_CLANG_PLUGIN=1
 
 if [ -n "$UPLOAD_PATH" ]; then
-    DIAGNOSTICS_DIR="$(cmd.exe //e:on //c if not exist ${UPLOAD_PATH} mkdir ${UPLOAD_PATH} && cd ${UPLOAD_PATH} && pwd)"
+    case "$(uname -s)" in
+    MINGW*)
+        DIAGNOSTICS_DIR="$(cmd.exe //e:on //c if not exist ${UPLOAD_PATH} mkdir ${UPLOAD_PATH} && cd ${UPLOAD_PATH} && pwd)"
+        ;;
+    *)
+        DIAGNOSTICS_DIR="${UPLOAD_PATH}"
+        ;;
+    esac
     export CFLAGS="$CFLAGS -fcrash-diagnostics-dir=${DIAGNOSTICS_DIR}"
     export CXXFLAGS="$CXXFLAGS -fcrash-diagnostics-dir=${DIAGNOSTICS_DIR}"
 fi
diff --git a/build/mozconfig.win-common b/build/mozconfig.win-common
index 6bf3254a10cf..8fcec491fbe1 100644
--- a/build/mozconfig.win-common
+++ b/build/mozconfig.win-common
@@ -4,7 +4,7 @@
 
 TOOLTOOL_DIR=${TOOLTOOL_DIR:-$topsrcdir}
 export MAKECAB=$TOOLTOOL_DIR/makecab.exe
-export DUMP_SYMS="${MOZ_FETCHES_DIR}/dump_syms/dump_syms.exe"
+export DUMP_SYMS="${MOZ_FETCHES_DIR}/dump_syms/dump_syms"
 
 if [ -z "$USE_ARTIFACT" ]; then
     if [ -n "$TASKCLUSTER_PGO_PROFILE_USE" ]; then
diff --git a/build/win64/mozconfig.vs2017 b/build/win64/mozconfig.vs2017
index 17938efdc58d..d19cb5a40ba0 100644
--- a/build/win64/mozconfig.vs2017
+++ b/build/win64/mozconfig.vs2017
@@ -4,7 +4,7 @@ if [ -z "${VSPATH}" ]; then
 fi
 
 if [ -d "${VSPATH}" ]; then
-    VSWINPATH="$(cd ${VSPATH} && pwd -W)"
+    VSWINPATH="$(cd ${VSPATH} && (pwd -W 2>/dev/null || pwd))"
 
     export WINDOWSSDKDIR="${VSWINPATH}/SDK"
     export WIN32_REDIST_DIR=${VSPATH}/VC/redist/x64/Microsoft.VC141.CRT
diff --git a/taskcluster/ci/build/windows.yml b/taskcluster/ci/build/windows.yml
index fd5400753db3..a8d83864f4a5 100755
--- a/taskcluster/ci/build/windows.yml
+++ b/taskcluster/ci/build/windows.yml
@@ -275,6 +275,64 @@ win64-plain/opt:
             - win64-node
             - win64-cbindgen
 
+win64-cross/opt:
+    description: "Win64 Opt Cross"
+    index:
+        product: firefox
+        job-name: win64-cross-opt
+    treeherder:
+        platform: windows2012-64/opt
+        symbol: Bx
+        tier: 2
+    worker-type: b-linux
+    worker:
+        docker-image: {in-tree: debian9-amd64-build}
+        max-run-time: 7200
+        env:
+            PERFHERDER_EXTRA_OPTIONS: cross
+            TOOLTOOL_MANIFEST: "browser/config/tooltool-manifests/win64/releng.manifest"
+    run:
+        actions: [get-secrets, build]
+        options: [append-env-variables-from-configs]
+        script: mozharness/scripts/fx_desktop_build.py
+        secrets: true
+        config:
+            - builds/releng_base_firefox.py
+              builds/releng_base_linux_64_builds.py
+        extra-config:
+            disable_package_metrics: true
+            mozconfig_platform: win64
+            mozconfig_variant: cross
+            env:
+                # Setting LD_PRELOAD at the worker level would set it during
+                # fetch-content, which can fail randomly when a tar/unzip subprocess
+                # of fetch-content starts while the library is being extracted by
+                # another.
+                LD_PRELOAD: "/builds/worker/fetches/liblowercase/liblowercase.so"
+                LOWERCASE_DIRS: "/builds/worker/workspace/build/src/vs2017_15.8.4/"
+        mozconfig-variant: cross
+    run-on-projects: ['try']
+    dependencies:
+        win64-clang-cl: toolchain-win64-clang-cl
+    fetches:
+        toolchain:
+            - linux64-binutils
+            - linux64-clang
+            - linux64-rust-cross-1.41
+            - linux64-nasm
+            - linux64-node
+            - linux64-cbindgen
+            - linux64-dump-syms
+            - linux64-wine
+            - linux64-liblowercase
+        fetch:
+            - nsis-3.01-win
+            - upx-3.95-win
+        win64-clang-cl:
+            - artifact: clang.tar.bz2
+              extract: true
+              dest: clang-cl
+
 win32-shippable/opt:
     description: "Win32 Opt Shippable"
     use-pgo: true