diff --git a/python/mozbuild/mozpack/copier.py b/python/mozbuild/mozpack/copier.py
index 308d3e6b0ea2..0d0fc09b9943 100644
--- a/python/mozbuild/mozpack/copier.py
+++ b/python/mozbuild/mozpack/copier.py
@@ -296,7 +296,7 @@ class FileCopier(FileRegistry):
 
         Returns a FileCopyResult that details what changed.
         '''
-        assert isinstance(destination, basestring)
+        assert isinstance(destination, six.string_types)
         assert not os.path.exists(destination) or os.path.isdir(destination)
 
         result = FileCopyResult()
@@ -565,7 +565,7 @@ class Jarrer(FileRegistry, BaseFile):
             def exists(self):
                 return self.deflater is not None
 
-        if isinstance(dest, basestring):
+        if isinstance(dest, six.string_types):
             dest = Dest(dest)
         assert isinstance(dest, Dest)
 
diff --git a/python/mozbuild/mozpack/files.py b/python/mozbuild/mozpack/files.py
index 827d338f975a..542103fb6e3d 100644
--- a/python/mozbuild/mozpack/files.py
+++ b/python/mozbuild/mozpack/files.py
@@ -177,7 +177,7 @@ class BaseFile(object):
         disabled when skip_if_older is False.
         Returns whether a copy was actually performed (True) or not (False).
         '''
-        if isinstance(dest, basestring):
+        if isinstance(dest, six.string_types):
             dest = Dest(dest)
         else:
             assert isinstance(dest, Dest)
@@ -297,11 +297,11 @@ class ExecutableFile(File):
 
     def copy(self, dest, skip_if_older=True):
         real_dest = dest
-        if not isinstance(dest, basestring):
+        if not isinstance(dest, six.string_types):
             fd, dest = mkstemp()
             os.close(fd)
             os.remove(dest)
-        assert isinstance(dest, basestring)
+        assert isinstance(dest, six.string_types)
         # If File.copy didn't actually copy because dest is newer, check the
         # file sizes. If dest is smaller, it means it is already stripped and
         # elfhacked and xz_compressed, so we can skip.
@@ -340,7 +340,7 @@ class AbsoluteSymlinkFile(File):
         File.__init__(self, path)
 
     def copy(self, dest, skip_if_older=True):
-        assert isinstance(dest, basestring)
+        assert isinstance(dest, six.string_types)
 
         # The logic in this function is complicated by the fact that symlinks
         # aren't universally supported. So, where symlinks aren't supported, we
@@ -431,7 +431,7 @@ class HardlinkFile(File):
     '''
 
     def copy(self, dest, skip_if_older=True):
-        assert isinstance(dest, basestring)
+        assert isinstance(dest, six.string_types)
 
         if not hasattr(os, 'link'):
             return super(HardlinkFile, self).copy(
@@ -493,7 +493,7 @@ class ExistingFile(BaseFile):
         self.required = required
 
     def copy(self, dest, skip_if_older=True):
-        if isinstance(dest, basestring):
+        if isinstance(dest, six.string_types):
             dest = Dest(dest)
         else:
             assert isinstance(dest, Dest)
@@ -540,7 +540,7 @@ class PreprocessedFile(BaseFile):
         '''
         Invokes the preprocessor to create the destination file.
         '''
-        if isinstance(dest, basestring):
+        if isinstance(dest, six.string_types):
             dest = Dest(dest)
         else:
             assert isinstance(dest, Dest)
diff --git a/python/mozbuild/mozpack/mozjar.py b/python/mozbuild/mozpack/mozjar.py
index fe512617136f..7b16b11ff8e1 100644
--- a/python/mozbuild/mozpack/mozjar.py
+++ b/python/mozbuild/mozpack/mozjar.py
@@ -67,7 +67,7 @@ class JarStruct(object):
     (deserialized), or with empty fields.
     '''
 
-    TYPE_MAPPING = {'uint32': ('I', 4), 'uint16': ('H', 2)}
+    TYPE_MAPPING = {'uint32': (b'I', 4), 'uint16': (b'H', 2)}
 
     def __init__(self, data=None):
         '''
@@ -627,7 +627,7 @@ class JarWriter(object):
             deflater = data
         else:
             deflater = Deflater(compress, compress_level=self._compress_level)
-            if isinstance(data, basestring):
+            if isinstance(data, (six.binary_type, six.string_types)):
                 deflater.write(data)
             elif hasattr(data, 'read'):
                 if hasattr(data, 'seek'):
diff --git a/python/mozbuild/mozpack/test/test_mozjar.py b/python/mozbuild/mozpack/test/test_mozjar.py
index a595a9efa724..e70f432e20b1 100644
--- a/python/mozbuild/mozpack/test/test_mozjar.py
+++ b/python/mozbuild/mozpack/test/test_mozjar.py
@@ -75,8 +75,8 @@ class TestJarStruct(unittest.TestCase):
         self.assertEqual(foo['qux'], 0x01ef)
         self.assertFalse('length' in foo)
         self.assertFalse('length2' in foo)
-        self.assertEqual(foo['string'], '012345')
-        self.assertEqual(foo['string2'], '67')
+        self.assertEqual(foo['string'], b'012345')
+        self.assertEqual(foo['string2'], b'67')
 
     def test_read_jar_struct(self):
         data = b'\x00\x04\x03\x02\x01\x42\x43\x44\x45\xab\xcd\xef' + \
@@ -130,7 +130,7 @@ class TestDeflater(unittest.TestCase):
         self.assertFalse(deflater.compressed)
         self.assertEqual(deflater.uncompressed_size, 0)
         self.assertEqual(deflater.compressed_size, deflater.uncompressed_size)
-        self.assertEqual(deflater.compressed_data, '')
+        self.assertEqual(deflater.compressed_data, b'')
         self.assertEqual(deflater.crc32, 0)
 
 
@@ -151,23 +151,23 @@ class TestJar(unittest.TestCase):
 
         files = [j for j in JarReader(fileobj=s)]
 
-        self.assertEqual(files[0].filename, 'foo')
+        self.assertEqual(files[0].filename, b'foo')
         self.assertFalse(files[0].compressed)
         self.assertEqual(files[0].read(), b'foo')
 
-        self.assertEqual(files[1].filename, 'bar')
+        self.assertEqual(files[1].filename, b'bar')
         self.assertTrue(files[1].compressed)
         self.assertEqual(files[1].read(), b'aaaaaaaaaaaaanopqrstuvwxyz')
 
-        self.assertEqual(files[2].filename, 'baz/qux')
+        self.assertEqual(files[2].filename, b'baz/qux')
         self.assertFalse(files[2].compressed)
         self.assertEqual(files[2].read(), b'aaaaaaaaaaaaanopqrstuvwxyz')
 
         if os.sep == '\\':
-            self.assertEqual(files[3].filename, 'baz/backslash',
+            self.assertEqual(files[3].filename, b'baz/backslash',
                              'backslashes in filenames on Windows should get normalized')
         else:
-            self.assertEqual(files[3].filename, 'baz\\backslash',
+            self.assertEqual(files[3].filename, b'baz\\backslash',
                              'backslashes in filenames on POSIX platform are untouched')
 
         s = MockDest()
@@ -179,47 +179,47 @@ class TestJar(unittest.TestCase):
         jar = JarReader(fileobj=s)
         files = [j for j in jar]
 
-        self.assertEqual(files[0].filename, 'bar')
+        self.assertEqual(files[0].filename, b'bar')
         self.assertFalse(files[0].compressed)
         self.assertEqual(files[0].read(), b'aaaaaaaaaaaaanopqrstuvwxyz')
 
-        self.assertEqual(files[1].filename, 'foo')
+        self.assertEqual(files[1].filename, b'foo')
         self.assertFalse(files[1].compressed)
         self.assertEqual(files[1].read(), b'foo')
 
-        self.assertEqual(files[2].filename, 'baz/qux')
+        self.assertEqual(files[2].filename, b'baz/qux')
         self.assertTrue(files[2].compressed)
         self.assertEqual(files[2].read(), b'aaaaaaaaaaaaanopqrstuvwxyz')
 
-        self.assertTrue('bar' in jar)
-        self.assertTrue('foo' in jar)
-        self.assertFalse('baz' in jar)
-        self.assertTrue('baz/qux' in jar)
-        self.assertTrue(jar['bar'], files[1])
-        self.assertTrue(jar['foo'], files[0])
-        self.assertTrue(jar['baz/qux'], files[2])
+        self.assertTrue(b'bar' in jar)
+        self.assertTrue(b'foo' in jar)
+        self.assertFalse(b'baz' in jar)
+        self.assertTrue(b'baz/qux' in jar)
+        self.assertTrue(jar[b'bar'], files[1])
+        self.assertTrue(jar[b'foo'], files[0])
+        self.assertTrue(jar[b'baz/qux'], files[2])
 
         s.seek(0)
         jar = JarReader(fileobj=s)
-        self.assertTrue('bar' in jar)
-        self.assertTrue('foo' in jar)
-        self.assertFalse('baz' in jar)
-        self.assertTrue('baz/qux' in jar)
+        self.assertTrue(b'bar' in jar)
+        self.assertTrue(b'foo' in jar)
+        self.assertFalse(b'baz' in jar)
+        self.assertTrue(b'baz/qux' in jar)
 
         files[0].seek(0)
-        self.assertEqual(jar['bar'].filename, files[0].filename)
-        self.assertEqual(jar['bar'].compressed, files[0].compressed)
-        self.assertEqual(jar['bar'].read(), files[0].read())
+        self.assertEqual(jar[b'bar'].filename, files[0].filename)
+        self.assertEqual(jar[b'bar'].compressed, files[0].compressed)
+        self.assertEqual(jar[b'bar'].read(), files[0].read())
 
         files[1].seek(0)
-        self.assertEqual(jar['foo'].filename, files[1].filename)
-        self.assertEqual(jar['foo'].compressed, files[1].compressed)
-        self.assertEqual(jar['foo'].read(), files[1].read())
+        self.assertEqual(jar[b'foo'].filename, files[1].filename)
+        self.assertEqual(jar[b'foo'].compressed, files[1].compressed)
+        self.assertEqual(jar[b'foo'].read(), files[1].read())
 
         files[2].seek(0)
-        self.assertEqual(jar['baz/qux'].filename, files[2].filename)
-        self.assertEqual(jar['baz/qux'].compressed, files[2].compressed)
-        self.assertEqual(jar['baz/qux'].read(), files[2].read())
+        self.assertEqual(jar[b'baz/qux'].filename, files[2].filename)
+        self.assertEqual(jar[b'baz/qux'].compressed, files[2].compressed)
+        self.assertEqual(jar[b'baz/qux'].read(), files[2].read())
 
     def test_rejar(self):
         s = MockDest()
@@ -236,15 +236,15 @@ class TestJar(unittest.TestCase):
         jar = JarReader(fileobj=new)
         files = [j for j in jar]
 
-        self.assertEqual(files[0].filename, 'foo')
+        self.assertEqual(files[0].filename, b'foo')
         self.assertFalse(files[0].compressed)
         self.assertEqual(files[0].read(), b'foo')
 
-        self.assertEqual(files[1].filename, 'bar')
+        self.assertEqual(files[1].filename, b'bar')
         self.assertTrue(files[1].compressed)
         self.assertEqual(files[1].read(), b'aaaaaaaaaaaaanopqrstuvwxyz')
 
-        self.assertEqual(files[2].filename, 'baz/qux')
+        self.assertEqual(files[2].filename, b'baz/qux')
         self.assertTrue(files[2].compressed)
         self.assertEqual(files[2].read(), b'aaaaaaaaaaaaanopqrstuvwxyz')
 
@@ -258,7 +258,7 @@ class TestJar(unittest.TestCase):
         jar = JarReader(fileobj=s)
         files = [j for j in jar]
 
-        self.assertEqual(files[0].filename, 'test_data')
+        self.assertEqual(files[0].filename, b'test_data')
         self.assertFalse(files[0].compressed)
         self.assertEqual(files[0].read(), b'test_data')
 
@@ -278,15 +278,15 @@ class TestPreload(unittest.TestCase):
             jar.add('foo', b'foo')
             jar.add('bar', b'abcdefghijklmnopqrstuvwxyz')
             jar.add('baz/qux', b'aaaaaaaaaaaaanopqrstuvwxyz')
-            jar.preload(['baz/qux', b'bar'])
+            jar.preload(['baz/qux', 'bar'])
 
         jar = JarReader(fileobj=s)
         self.assertEqual(jar.last_preloaded, b'bar')
         files = [j for j in jar]
 
-        self.assertEqual(files[0].filename, 'baz/qux')
-        self.assertEqual(files[1].filename, 'bar')
-        self.assertEqual(files[2].filename, 'foo')
+        self.assertEqual(files[0].filename, b'baz/qux')
+        self.assertEqual(files[1].filename, b'bar')
+        self.assertEqual(files[2].filename, b'foo')
 
 
 class TestJarLog(unittest.TestCase):