diff --git a/dom/base/usecounters.py b/dom/base/usecounters.py
--- a/dom/base/usecounters.py
+++ b/dom/base/usecounters.py
@@ -6,11 +6,11 @@
 import re
 
 
 def read_conf(conf_filename):
     # Can't read/write from a single StringIO, so make a new one for reading.
-    stream = open(conf_filename, "rU")
+    stream = open(conf_filename, "r")
 
     def parse_counters(stream):
         for line_num, line in enumerate(stream):
             line = line.rstrip("\n")
             if not line or line.startswith("//"):
diff --git a/python/mozbuild/mozbuild/action/process_define_files.py b/python/mozbuild/mozbuild/action/process_define_files.py
--- a/python/mozbuild/mozbuild/action/process_define_files.py
+++ b/python/mozbuild/mozbuild/action/process_define_files.py
@@ -34,11 +34,11 @@
     if mozpath.basedir(
         path, [mozpath.join(topsrcdir, "js/src")]
     ) and not config.substs.get("JS_STANDALONE"):
         config = PartialConfigEnvironment(mozpath.join(topobjdir, "js", "src"))
 
-    with open(path, "rU") as input:
+    with open(path, "r") as input:
         r = re.compile(
             "^\s*#\s*(?P<cmd>[a-z]+)(?:\s+(?P<name>\S+)(?:\s+(?P<value>\S+))?)?", re.U
         )
         for l in input:
             m = r.match(l)
diff --git a/python/mozbuild/mozbuild/backend/base.py b/python/mozbuild/mozbuild/backend/base.py
--- a/python/mozbuild/mozbuild/backend/base.py
+++ b/python/mozbuild/mozbuild/backend/base.py
@@ -270,11 +270,11 @@
         self._write_purgecaches(config)
 
         return status
 
     @contextmanager
-    def _write_file(self, path=None, fh=None, readmode="rU"):
+    def _write_file(self, path=None, fh=None, readmode="r"):
         """Context manager to write a file.
 
         This is a glorified wrapper around FileAvoidWrite with integration to
         update the summary data on this instance.
 
diff --git a/python/mozbuild/mozbuild/preprocessor.py b/python/mozbuild/mozbuild/preprocessor.py
--- a/python/mozbuild/mozbuild/preprocessor.py
+++ b/python/mozbuild/mozbuild/preprocessor.py
@@ -529,11 +529,11 @@
                 )
             depfile = get_output_file(options.depend)
 
         if args:
             for f in args:
-                with io.open(f, "rU", encoding="utf-8") as input:
+                with io.open(f, "r", encoding="utf-8") as input:
                     self.processFile(input=input, output=out)
             if depfile:
                 mk = Makefile()
                 mk.create_rule([six.ensure_text(options.output)]).add_dependencies(
                     self.includes
@@ -858,11 +858,11 @@
                 args = _to_text(args)
                 if filters:
                     args = self.applyFilters(args)
                 if not os.path.isabs(args):
                     args = os.path.join(self.curdir, args)
-                args = io.open(args, "rU", encoding="utf-8")
+                args = io.open(args, "r", encoding="utf-8")
             except Preprocessor.Error:
                 raise
             except Exception:
                 raise Preprocessor.Error(self, "FILE_NOT_FOUND", _to_text(args))
         self.checkLineNumbers = bool(
@@ -912,11 +912,11 @@
 
 
 def preprocess(includes=[sys.stdin], defines={}, output=sys.stdout, marker="#"):
     pp = Preprocessor(defines=defines, marker=marker)
     for f in includes:
-        with io.open(f, "rU", encoding="utf-8") as input:
+        with io.open(f, "r", encoding="utf-8") as input:
             pp.processFile(input=input, output=output)
     return pp.includes
 
 
 # Keep this module independently executable.
diff --git a/python/mozbuild/mozbuild/util.py b/python/mozbuild/mozbuild/util.py
--- a/python/mozbuild/mozbuild/util.py
+++ b/python/mozbuild/mozbuild/util.py
@@ -234,11 +234,11 @@
     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
diff --git a/python/mozbuild/mozpack/files.py b/python/mozbuild/mozpack/files.py
--- a/python/mozbuild/mozpack/files.py
+++ b/python/mozbuild/mozpack/files.py
@@ -552,11 +552,11 @@
 
     def inputs(self):
         pp = Preprocessor(defines=self.defines, marker=self.marker)
         pp.setSilenceDirectiveWarnings(self.silence_missing_directive_warnings)
 
-        with _open(self.path, "rU") as input:
+        with _open(self.path, "r") as input:
             with _open(os.devnull, "w") as output:
                 pp.processFile(input=input, output=output)
 
         # This always yields at least self.path.
         return pp.includes
@@ -609,11 +609,11 @@
         if self.depfile:
             deps_out = FileAvoidWrite(self.depfile)
         pp = Preprocessor(defines=self.defines, marker=self.marker)
         pp.setSilenceDirectiveWarnings(self.silence_missing_directive_warnings)
 
-        with _open(self.path, "rU") as input:
+        with _open(self.path, "r") as input:
             pp.processFile(input=input, output=dest, depfile=deps_out)
 
         dest.close()
         if self.depfile:
             deps_out.close()