--- a/mach
+++ b/mach
@@ -28,18 +28,17 @@ for dir_path in ancestors(os.getcwd()):
         import json
         info = json.load(open(mozinfo_path))
         if "mozconfig" in info and "MOZCONFIG" not in os.environ:
             # If the MOZCONFIG environment variable is not already set, set it
             # to the value from mozinfo.json.  This will tell the build system
             # 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:
             # Continue searching for mach_bootstrap in the source directory.
             dir_path = info["topsrcdir"]
 
     # If we find the mach bootstrap module, we are in the srcdir.
     mach_path = os.path.join(dir_path, "build/mach_bootstrap.py")

--- a/python/mach_commands.py
+++ b/python/mach_commands.py
@@ -54,17 +54,18 @@ class MachCommands(MachCommandBase):
     @Command('python', category='devenv',
         allow_all_args=True,
         description='Run Python.')
     @CommandArgument('args', nargs=argparse.REMAINDER)
     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.')
     @CommandArgument('--verbose',
         default=False,
         action='store_true',
         help='Verbose output.')
     @CommandArgument('--stop',
@@ -104,17 +105,18 @@ class MachCommands(MachCommandBase):
             def _line_handler(line):
                 if not file_displayed_test and line.startswith('TEST-'):
                     file_displayed_test.append(True)
 
             inner_return_code = self.run_process(
                 [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
 
             if not file_displayed_test:
                 self.log(logging.WARN, 'python-test', {'file': file},
                          'TEST-UNEXPECTED-FAIL | No test output (missing mozunit.main() call?): {file}')
 
             if verbose:

--- a/testing/mochitest/mach_commands.py
+++ b/testing/mochitest/mach_commands.py
@@ -144,17 +144,16 @@ class MochitestRunner(MozbuildObject):
             test_root = runner.getTestRoot(options)
             test_root_file = mozpack.path.join(mochitest_dir, test_root, test_path)
             if not os.path.exists(test_root_file):
                 print('Specified test path does not exist: %s' % test_root_file)
                 print('You may need to run |mach build| to build the test files.')
                 return 1
 
             options.testPath = test_path
-            env = {'TEST_PATH': test_path}
 
         if rerun_failures:
             options.testManifest = failure_file_path
 
         if debugger:
             options.debugger = debugger
 
         if debugger_args: