--- a/python/mozbuild/mozbuild/action/langpack_manifest.py
+++ b/python/mozbuild/mozbuild/action/langpack_manifest.py
@@ -13,16 +13,17 @@ from __future__ import absolute_import
 import argparse
 import sys
 import os
 import json
 import io
 import datetime
 import requests
 import mozversioncontrol
+import mozpack.path as mozpath
 from mozpack.chrome.manifest import (
     Manifest,
     ManifestLocale,
     parse_manifest,
 )
 from mozbuild.preprocessor import Preprocessor
 
 
@@ -262,28 +263,29 @@ def parse_chrome_manifest(path, base_pat
     for entry in parse_manifest(None, path):
         if isinstance(entry, Manifest):
             parse_chrome_manifest(
                 os.path.join(os.path.dirname(path), entry.relpath),
                 base_path,
                 chrome_entries
             )
         elif isinstance(entry, ManifestLocale):
+            entry_path = os.path.join(
+                os.path.relpath(
+                    os.path.dirname(path),
+                    base_path
+                ),
+                entry.relpath
+            )
             chrome_entries.append({
                 'type': 'locale',
                 'alias': entry.name,
                 'locale': entry.id,
                 'platforms': convert_entry_flags_to_platform_codes(entry.flags),
-                'path': os.path.join(
-                    os.path.relpath(
-                        os.path.dirname(path),
-                        base_path
-                    ),
-                    entry.relpath
-                )
+                'path': mozpath.normsep(entry_path)
             })
         else:
             raise Exception('Unknown type {0}'.format(entry.name))
 
 
 ###
 # Generates a new web manifest dict with values specific for a language pack.
 #