--- seamonkey-2.53.16/python/mozbuild/mozbuild/util.py
+++ seamonkey-2.53.16/python/mozbuild/mozbuild/util.py
@@ -222,17 +222,17 @@
     enabled by default because it a) doesn't make sense for binary files b)
     could add unwanted overhead to calls.
 
     Additionally, there is dry run mode where the file is not actually written
     out, but reports whether the file was existing and would have been updated
     still occur, as well as diff capture if requested.
     """
 
-    def __init__(self, filename, capture_diff=False, dry_run=False, readmode='rU'):
+    def __init__(self, filename, capture_diff=False, dry_run=False, readmode='r'):
         BytesIO.__init__(self)
         self.name = filename
         assert type(capture_diff) == bool
         assert type(dry_run) == bool
         assert 'r' in readmode
         self._capture_diff = capture_diff
         self._write_to_file = not dry_run
         self.diff = None