--- a/python/mozbuild/mozbuild/base.py
+++ b/python/mozbuild/mozbuild/base.py
@@ -154,52 +154,27 @@ class MozbuildObject(ProcessExecutionMix
             topsrcdir, topobjdir, mozconfig = load_mozinfo(mozinfo_path)
 
         # If we were successful, we're only guaranteed to find a topsrcdir. If
         # we couldn't find that, there's nothing we can do.
         if not topsrcdir:
             raise BuildEnvironmentNotFoundException(
                 'Could not find Mozilla source tree or build environment.')
 
-        # Now we try to load the config for this environment. If mozconfig is
-        # None, read_mozconfig() will attempt to find one in the existing
-        # environment. If no mozconfig is present, the config will not have
-        # much defined.
-        loader = MozconfigLoader(topsrcdir)
-        current_project = os.environ.get('MOZ_CURRENT_PROJECT')
-        config = loader.read_mozconfig(mozconfig, moz_build_app=current_project)
-
-        config_topobjdir = MozbuildObject.resolve_mozconfig_topobjdir(
-            topsrcdir, config)
-
-        # If we're inside a objdir and the found mozconfig resolves to
-        # another objdir, we abort. The reasoning here is that if you are
-        # inside an objdir you probably want to perform actions on that objdir,
-        # not another one. This prevents accidental usage of the wrong objdir
-        # when the current objdir is ambiguous.
-        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 = 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.
+        # If we can't resolve topobjdir, oh well. We'll figure out when we need
+        # one.
         return cls(topsrcdir, None, None, topobjdir=topobjdir,
                    mozconfig=mozconfig)
 
     @staticmethod
     def resolve_mozconfig_topobjdir(topsrcdir, mozconfig, default=None):
         topobjdir = mozconfig['topobjdir'] or default
         if not topobjdir:
             return None
@@ -232,17 +207,17 @@ class MozbuildObject(ProcessExecutionMix
         return self._virtualenv_manager
 
     @property
     def mozconfig(self):
         """Returns information about the current mozconfig file.
 
         This a dict as returned by MozconfigLoader.read_mozconfig()
         """
-        if self._mozconfig is MozconfigLoader.AUTODETECT:
+        if not isinstance(self._mozconfig, dict):
             loader = MozconfigLoader(self.topsrcdir)
             self._mozconfig = loader.read_mozconfig(path=self._mozconfig,
                 moz_build_app=os.environ.get('MOZ_CURRENT_PROJECT'))
 
         return self._mozconfig
 
     @property
     def config_environment(self):
@@ -694,16 +669,27 @@ class MachCommandBase(MozbuildObject):
         if hasattr(context, 'detect_virtualenv_mozinfo'):
             detect_virtualenv_mozinfo = getattr(context,
                 'detect_virtualenv_mozinfo')
         try:
             dummy = MozbuildObject.from_environment(cwd=context.cwd,
                 detect_virtualenv_mozinfo=detect_virtualenv_mozinfo)
             topsrcdir = dummy.topsrcdir
             topobjdir = dummy._topobjdir
+            if topobjdir:
+                # If we're inside a objdir and the found mozconfig resolves to
+                # another objdir, we abort. The reasoning here is that if you
+                # are inside an objdir you probably want to perform actions on
+                # that objdir, not another one. This prevents accidental usage
+                # of the wrong objdir when the current objdir is ambiguous.
+                config_topobjdir = MozbuildObject.resolve_mozconfig_topobjdir(
+                    topsrcdir, dummy.mozconfig)
+                if config_topobjdir and not samepath(topobjdir,
+                                                     config_topobjdir):
+                    raise ObjdirMismatchException(topobjdir, config_topobjdir)
         except BuildEnvironmentNotFoundException:
             pass
         except ObjdirMismatchException as e:
             print('Ambiguous object directory detected. We detected that '
                 'both %s and %s could be object directories. This is '
                 'typically caused by having a mozconfig pointing to a '
                 'different object directory from the current working '
                 'directory. To solve this problem, ensure you do not have a '

--- a/python/mozbuild/mozbuild/test/test_base.py
+++ b/python/mozbuild/mozbuild/test/test_base.py
@@ -7,16 +7,17 @@ from __future__ import unicode_literals
 import json
 import os
 import shutil
 import subprocess
 import sys
 import tempfile
 import unittest
 
+from cStringIO import StringIO
 from mozfile.mozfile import NamedTemporaryFile
 
 from mozunit import main
 
 from mach.logging import LoggingManager
 
 from mozbuild.base import (
     BadEnvironmentException,
@@ -267,18 +268,36 @@ class TestMozbuildObject(unittest.TestCa
             with open(mozinfo, 'wt') as fh:
                 json.dump(dict(
                     topsrcdir=topsrcdir,
                     mozconfig=mozconfig,
                 ), fh)
 
             os.chdir(topobjdir)
 
-            with self.assertRaises(ObjdirMismatchException):
-                MozbuildObject.from_environment(detect_virtualenv_mozinfo=False)
+            class MockMachContext(object):
+                pass
+
+            context = MockMachContext()
+            context.cwd = topobjdir
+            context.topdir = topsrcdir
+            context.settings = None
+            context.log_manager = None
+            context.detect_virtualenv_mozinfo=False
+
+            stdout = sys.stdout
+            sys.stdout = StringIO()
+            try:
+                with self.assertRaises(SystemExit):
+                    MachCommandBase(context)
+
+                self.assertTrue(sys.stdout.getvalue().startswith(
+                    'Ambiguous object directory detected.'))
+            finally:
+                sys.stdout = stdout
 
         finally:
             os.chdir(self._old_cwd)
             shutil.rmtree(d)
 
     def test_config_guess(self):
         # It's difficult to test for exact values from the output of
         # config.guess because they vary depending on platform.