--- a/dom/bindings/mozwebidlcodegen/test/test_mozwebidlcodegen.py
+++ b/dom/bindings/mozwebidlcodegen/test/test_mozwebidlcodegen.py
@@ -234,17 +234,17 @@ class TestWebIDLCodegenManager(unittest.
         # We don't want to mutate files in the source directory because we want
         # to be able to build from a read-only filesystem. So, we install a
         # dummy module and rewrite the metadata to say it comes from the source
         # directory.
         #
         # Hacking imp to accept a MockedFile doesn't appear possible. So for
         # the first iteration we read from a temp file. The second iteration
         # doesn't need to import, so we are fine with a mocked file.
-        fake_path = mozpath.join(OUR_DIR, 'fakemodule.py')
+        fake_path = os.path.normpath(mozpath.join(OUR_DIR, 'fakemodule.py'))
         with NamedTemporaryFile('wt') as fh:
             fh.write('# Original content')
             fh.flush()
             mod = imp.load_source('mozwebidlcodegen.fakemodule', fh.name)
             mod.__file__ = fake_path
 
             args = self._get_manager_args()
             m1 = WebIDLCodegenManager(**args)