--- a/dom/base/usecounters.py
+++ b/dom/base/usecounters.py
@@ -3,17 +3,17 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import collections
 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("//"):
                 # empty line or comment
                 continue
             m = re.match(r"method ([A-Za-z0-9]+)\.([A-Za-z0-9]+)$", line)

--- a/python/mozbuild/mozbuild/action/process_define_files.py
+++ b/python/mozbuild/mozbuild/action/process_define_files.py
@@ -31,17 +31,17 @@ def process_define_file(output, input):
 
     config = PartialConfigEnvironment(topobjdir)
 
     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)
             if m:
                 cmd = m.group("cmd")
                 name = m.group("name")

--- a/python/mozbuild/mozbuild/backend/base.py
+++ b/python/mozbuild/mozbuild/backend/base.py
@@ -267,17 +267,17 @@ class BuildBackend(LoggingMixin):
         If an exception is raised, |mach build| will fail with a
         non-zero exit code.
         """
         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.
 
         Example usage:
 
             with self._write_file('foo.txt') as fh:

--- a/python/mozbuild/mozbuild/preprocessor.py
+++ b/python/mozbuild/mozbuild/preprocessor.py
@@ -526,17 +526,17 @@ class Preprocessor:
             if not options.output:
                 raise Preprocessor.Error(
                     self, "--depend doesn't work with stdout", None
                 )
             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
                 )
                 mk.dump(depfile)
                 depfile.close()
@@ -855,17 +855,17 @@ class Preprocessor:
         self.checkLineNumbers = False
         if isName:
             try:
                 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(
             re.search("\.(js|jsm|java|webidl)(?:\.in)?$", args.name)
         )
         oldFile = self.context["FILE"]
@@ -909,17 +909,17 @@ class Preprocessor:
 
     def do_error(self, args):
         raise Preprocessor.Error(self, "Error: ", _to_text(args))
 
 
 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.
 if __name__ == "__main__":
     pp = Preprocessor()
     pp.handleCommandLine(None, True)

--- a/python/mozbuild/mozbuild/util.py
+++ b/python/mozbuild/mozbuild/util.py
@@ -231,17 +231,17 @@ class FileAvoidWrite(BytesIO):
     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

--- a/python/mozbuild/mozpack/files.py
+++ b/python/mozbuild/mozpack/files.py
@@ -549,17 +549,17 @@ class PreprocessedFile(BaseFile):
         self.defines = defines
         self.extra_depends = list(extra_depends or [])
         self.silence_missing_directive_warnings = silence_missing_directive_warnings
 
     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
 
     def copy(self, dest, skip_if_older=True):
         """
@@ -606,17 +606,17 @@ class PreprocessedFile(BaseFile):
             return False
 
         deps_out = None
         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()
 
         return True