diff --git a/mach b/mach
index 9aba2825387e..dbbd8fca40e2 100755
--- a/mach
+++ b/mach
@@ -33,8 +33,7 @@ for dir_path in ancestors(os.getcwd()):
             # to look for a config file at the path in $MOZCONFIG rather than
             # its default locations.
             #
-            # Note: subprocess requires native strings in os.environ Python
-            # 2.7.2 and earlier on Windows.
+            # Note: subprocess requires native strings in os.environ on Windows
             os.environ[b"MOZCONFIG"] = str(info["mozconfig"])
 
         if "topsrcdir" in info:
diff --git a/python/mach_commands.py b/python/mach_commands.py
index 50286ba232f9..3d920b0e6c1a 100644
--- a/python/mach_commands.py
+++ b/python/mach_commands.py
@@ -59,7 +59,8 @@ def python(self, args):
         return self.run_process([self.python_executable] + args,
             pass_thru=True, # Allow user to run Python interactively.
             ensure_exit_code=False, # Don't throw on non-zero exit code.
-            append_env={'PYTHONDONTWRITEBYTECODE': '1'})
+            # Note: subprocess requires native strings in os.environ on Windows
+            append_env={b'PYTHONDONTWRITEBYTECODE': str('1')})
 
     @Command('python-test', category='testing',
         description='Run Python unit tests.')
@@ -109,7 +110,8 @@ def _line_handler(line):
                 [self.python_executable, file],
                 ensure_exit_code=False, # Don't throw on non-zero exit code.
                 log_name='python-test',
-                append_env={'PYTHONDONTWRITEBYTECODE': '1'},
+                # subprocess requires native strings in os.environ on Windows
+                append_env={b'PYTHONDONTWRITEBYTECODE': str('1')},
                 line_handler=_line_handler)
             return_code += inner_return_code
 
diff --git a/testing/mochitest/mach_commands.py b/testing/mochitest/mach_commands.py
index 5e14e278ad17..7ec1afc08b8c 100644
--- a/testing/mochitest/mach_commands.py
+++ b/testing/mochitest/mach_commands.py
@@ -149,7 +149,6 @@ def run_mochitest_test(self, suite=None, test_file=None, debugger=None,
                 return 1
 
             options.testPath = test_path
-            env = {'TEST_PATH': test_path}
 
         if rerun_failures:
             options.testManifest = failure_file_path