diff --git a/python/mozbuild/mozbuild/base.py b/python/mozbuild/mozbuild/base.py
index 4933e5192df0..0ade46de2773 100644
--- a/python/mozbuild/mozbuild/base.py
+++ b/python/mozbuild/mozbuild/base.py
@@ -19,7 +19,6 @@ except ImportError:
     # shutil.which is not available in Python 2.7
     import which
 
-from StringIO import StringIO
 from mach.mixin.process import ProcessExecutionMixin
 from mozversioncontrol import (
     get_repository_from_build_config,
@@ -273,7 +272,7 @@ class MozbuildObject(ProcessExecutionMixin):
         # the environment variable, which has an impact on autodetection (when
         # path is MozconfigLoader.AUTODETECT), and memoization wouldn't account
         # for it without the explicit (unused) argument.
-        out = StringIO()
+        out = six.StringIO()
         env = os.environ
         if path and path != MozconfigLoader.AUTODETECT:
             env = dict(env)
diff --git a/python/mozbuild/mozpack/test/test_files.py b/python/mozbuild/mozpack/test/test_files.py
index d45e653e95fb..6daa360e01dc 100644
--- a/python/mozbuild/mozpack/test/test_files.py
+++ b/python/mozbuild/mozpack/test/test_files.py
@@ -60,12 +60,12 @@ import mozfile
 import mozunit
 import os
 import random
+import six
 import sys
 import tarfile
 import mozpack.path as mozpath
 from tempfile import mkdtemp
 from io import BytesIO
-from StringIO import StringIO
 
 
 class TestWithTmpDir(unittest.TestCase):
@@ -921,7 +921,7 @@ class TestMinifiedJavaScript(TestWithTmpDir):
 
     def test_minified_verify_failure(self):
         orig_f = GeneratedFile(b'\n'.join(self.orig_lines))
-        errors.out = StringIO()
+        errors.out = six.StringIO()
         min_f = MinifiedJavaScript(orig_f,
                                    verify_command=self._verify_command('1'))