--- a/python/mozbuild/mozbuild/config_status.py
+++ b/python/mozbuild/mozbuild/config_status.py
@@ -147,17 +147,17 @@ def config_status(topobjdir='.', topsrcd
     print('Reticulating splines...', file=sys.stderr)
     summary = the_backend.consume(definitions)
 
     for line in summary.summaries():
         print(line, file=sys.stderr)
 
     if options.diff:
         for path, diff in sorted(summary.file_diffs.items()):
-            print(diff)
+            print('\n'.join(diff))
 
     # Advertise Visual Studio if appropriate.
     if os.name == 'nt' and options.backend == 'RecursiveMake':
         print(VISUAL_STUDIO_ADVERTISEMENT)
 
     # Advertise Eclipse if it is appropriate.
     if MachCommandConditions.is_android(env):
         if options.backend == 'RecursiveMake':

--- a/python/mozbuild/mozbuild/test/test_util.py
+++ b/python/mozbuild/mozbuild/test/test_util.py
@@ -123,32 +123,32 @@ class TestFileAvoidWrite(unittest.TestCa
     def test_diff_update(self):
         """Diffs are produced on file update."""
 
         with MockedOpen({'file': 'old'}):
             faw = FileAvoidWrite('file', capture_diff=True)
             faw.write('new')
             faw.close()
 
-            self.assertIsInstance(faw.diff, unicode)
-            self.assertIn('-old', faw.diff)
-            self.assertIn('+new', faw.diff)
+            diff = '\n'.join(faw.diff)
+            self.assertIn('-old', diff)
+            self.assertIn('+new', diff)
 
     def test_diff_create(self):
         """Diffs are produced when files are created."""
 
         tmpdir = tempfile.mkdtemp()
         try:
             path = os.path.join(tmpdir, 'file')
             faw = FileAvoidWrite(path, capture_diff=True)
             faw.write('new')
             faw.close()
 
-            self.assertIsInstance(faw.diff, unicode)
-            self.assertIn('+new', faw.diff)
+            diff = '\n'.join(faw.diff)
+            self.assertIn('+new', diff)
         finally:
             shutil.rmtree(tmpdir)
 
 class TestResolveTargetToMake(unittest.TestCase):
     def setUp(self):
         self.topobjdir = data_path
 
     def assertResolve(self, path, expected):

--- a/python/mozbuild/mozbuild/util.py
+++ b/python/mozbuild/mozbuild/util.py
@@ -162,18 +162,18 @@ class FileAvoidWrite(StringIO):
         with open(self.name, 'w') as file:
             file.write(buf)
 
         if self._capture_diff:
             try:
                 old_lines = old_content.splitlines() if old_content else []
                 new_lines = buf.splitlines()
 
-                self.diff = '\n'.join(difflib.unified_diff(old_lines, new_lines,
-                    self.name, self.name, n=4, lineterm=''))
+                self.diff = difflib.unified_diff(old_lines, new_lines,
+                    self.name, self.name, n=4, lineterm='')
             # FileAvoidWrite isn't unicode/bytes safe. So, files with non-ascii
             # content or opened and written in different modes may involve
             # implicit conversion and this will make Python unhappy. Since
             # diffing isn't a critical feature, we just ignore the failure.
             # This can go away once FileAvoidWrite uses io.BytesIO and
             # io.StringIO. But that will require a lot of work.
             except (UnicodeDecodeError, UnicodeEncodeError):
                 self.diff = 'Binary or non-ascii file changed: %s' % self.name