--- a/python/mozbuild/mozbuild/base.py
+++ b/python/mozbuild/mozbuild/base.py
@@ -75,24 +75,24 @@ class MozbuildObject(ProcessExecutionMix
     """
     def __init__(self, topsrcdir, settings, log_manager, topobjdir=None):
         """Create a new Mozbuild object instance.
 
         Instances are bound to a source directory, a ConfigSettings instance,
         and a LogManager instance. The topobjdir may be passed in as well. If
         it isn't, it will be calculated from the active mozconfig.
         """
-        self.topsrcdir = topsrcdir
+        self.topsrcdir = mozpath.normsep(topsrcdir)
         self.settings = settings
 
         self.populate_logger()
         self.log_manager = log_manager
 
         self._make = None
-        self._topobjdir = topobjdir
+        self._topobjdir = mozpath.normsep(topobjdir) if topobjdir else topobjdir
         self._mozconfig = None
         self._config_guess_output = None
         self._config_environment = None
         self._virtualenv_manager = None
 
     @classmethod
     def from_environment(cls, cwd=None, detect_virtualenv_mozinfo=True):
         """Create a MozbuildObject by detecting the proper one from the env.
@@ -179,19 +179,20 @@ class MozbuildObject(ProcessExecutionMix
         if topobjdir and config_topobjdir:
             if current_project:
                 config_topobjdir = os.path.join(config_topobjdir, current_project)
 
             _config_topobjdir = config_topobjdir
             if not samepath(topobjdir, _config_topobjdir):
                 raise ObjdirMismatchException(topobjdir, _config_topobjdir)
 
+        topsrcdir = mozpath.normsep(topsrcdir)
         topobjdir = topobjdir or config_topobjdir
         if topobjdir:
-            topobjdir = os.path.normpath(topobjdir)
+            topobjdir = mozpath.normsep(os.path.normpath(topobjdir))
 
             if topsrcdir == topobjdir:
                 raise BadEnvironmentException('The object directory appears '
                     'to be the same as your source directory (%s). This build '
                     'configuration is not supported.' % topsrcdir)
 
         # If we can't resolve topobjdir, oh well. The constructor will figure
         # it out via config.guess.
@@ -205,17 +206,17 @@ class MozbuildObject(ProcessExecutionMix
 
         if '@CONFIG_GUESS@' in topobjdir:
             topobjdir = topobjdir.replace('@CONFIG_GUESS@',
                 MozbuildObject.resolve_config_guess(mozconfig, topsrcdir))
 
         if not os.path.isabs(topobjdir):
             topobjdir = os.path.abspath(os.path.join(topsrcdir, topobjdir))
 
-        return os.path.normpath(topobjdir)
+        return mozpath.normsep(os.path.normpath(topobjdir))
 
     @property
     def topobjdir(self):
         if self._topobjdir is None:
             self._topobjdir = MozbuildObject.resolve_mozconfig_topobjdir(
                 self.topsrcdir, self.mozconfig, default='obj-@CONFIG_GUESS@')
 
         return self._topobjdir

--- a/python/mozbuild/mozbuild/test/test_base.py
+++ b/python/mozbuild/mozbuild/test/test_base.py
@@ -23,16 +23,17 @@ from mozbuild.base import (
     MachCommandBase,
     MozbuildObject,
     ObjdirMismatchException,
     PathArgument,
 )
 
 from mozbuild.backend.configenvironment import ConfigEnvironment
 from buildconfig import topsrcdir, topobjdir
+import mozpack.path as mozpath
 
 
 curdir = os.path.dirname(__file__)
 log_manager = LoggingManager()
 
 
 class TestMozbuildObject(unittest.TestCase):
     def setUp(self):
@@ -54,28 +55,29 @@ class TestMozbuildObject(unittest.TestCa
 
         with NamedTemporaryFile() as mozconfig:
             os.environ[b'MOZCONFIG'] = mozconfig.name
 
             self.assertIsNotNone(base.topobjdir)
             self.assertEqual(len(base.topobjdir.split()), 1)
             self.assertTrue(base.topobjdir.endswith(base._config_guess))
             self.assertTrue(os.path.isabs(base.topobjdir))
-            self.assertTrue(base.topobjdir.startswith(topsrcdir))
+            print(base.topobjdir, base.topsrcdir)
+            self.assertTrue(base.topobjdir.startswith(base.topsrcdir))
 
     def test_objdir_trailing_slash(self):
         """Trailing slashes in topobjdir should be removed."""
         base = self.get_base()
 
         with NamedTemporaryFile() as mozconfig:
             mozconfig.write('mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/foo/')
             mozconfig.flush()
             os.environ[b'MOZCONFIG'] = mozconfig.name
 
-            self.assertEqual(base.topobjdir, os.path.join(base.topsrcdir,
+            self.assertEqual(base.topobjdir, mozpath.join(base.topsrcdir,
                 'foo'))
             self.assertTrue(base.topobjdir.endswith('foo'))
 
     def test_objdir_config_status(self):
         """Ensure @CONFIG_GUESS@ is handled when loading mozconfig."""
         base = self.get_base()
         cmd = base._normalize_command(
             [os.path.join(topsrcdir, 'build', 'autoconf', 'config.guess')],
@@ -108,30 +110,30 @@ class TestMozbuildObject(unittest.TestCa
                 ), fh)
 
             os.environ[b'MOZCONFIG'] = mozconfig.encode('utf-8')
             os.chdir(topobjdir)
 
             obj = MozbuildObject.from_environment(
                 detect_virtualenv_mozinfo=False)
 
-            self.assertEqual(obj.topobjdir, topobjdir)
+            self.assertEqual(obj.topobjdir, mozpath.normsep(topobjdir))
         finally:
             os.chdir(self._old_cwd)
             shutil.rmtree(d)
 
     def test_relative_objdir(self):
         """Relative defined objdirs are loaded properly."""
         d = os.path.realpath(tempfile.mkdtemp())
         try:
             mozconfig = os.path.join(d, 'mozconfig')
             with open(mozconfig, 'wt') as fh:
                 fh.write('mk_add_options MOZ_OBJDIR=./objdir')
 
-            topobjdir = os.path.join(d, 'objdir')
+            topobjdir = mozpath.join(d, 'objdir')
             os.mkdir(topobjdir)
 
             mozinfo = os.path.join(topobjdir, 'mozinfo.json')
             with open(mozinfo, 'wt') as fh:
                 json.dump(dict(
                     topsrcdir=d,
                     mozconfig=mozconfig,
                 ), fh)
@@ -211,18 +213,18 @@ class TestMozbuildObject(unittest.TestCa
             context.cwd = topobjdir
             context.topdir = topsrcdir
             context.settings = None
             context.log_manager = None
             context.detect_virtualenv_mozinfo=False
 
             o = MachCommandBase(context)
 
-            self.assertEqual(o.topobjdir, topobjdir)
-            self.assertEqual(o.topsrcdir, topsrcdir)
+            self.assertEqual(o.topobjdir, mozpath.normsep(topobjdir))
+            self.assertEqual(o.topsrcdir, mozpath.normsep(topsrcdir))
 
         finally:
             os.chdir(self._old_cwd)
             shutil.rmtree(d)
 
     def test_objdir_is_srcdir_rejected(self):
         """Ensure the srcdir configurations are rejected."""
         d = os.path.realpath(tempfile.mkdtemp())