--- a/testing/firefox-ui/mach_commands.py
+++ b/testing/firefox-ui/mach_commands.py
@@ -29,17 +29,17 @@ def setup_argument_parser_functional():
 def setup_argument_parser_update():
     from firefox_ui_harness.arguments.update import UpdateArguments
     from mozlog.structured import commandline
     parser = UpdateArguments()
     commandline.add_logging_group(parser)
     return parser
 
 
-def run_firefox_ui_test(testtype=None, topsrcdir=None, **kwargs):
+def run_firefox_ui_test(testtype=None, topsrcdir=None, topobjdir=None, **kwargs):
     from mozlog.structured import commandline
     from argparse import Namespace
     import firefox_ui_harness
 
     if testtype == 'functional':
         parser = setup_argument_parser_functional()
     else:
         parser = setup_argument_parser_update()
@@ -75,16 +75,19 @@ def run_firefox_ui_test(testtype=None, t
     elif not kwargs.get('tests'):
         # If no tests have been selected, set default ones
         kwargs['tests'] = [os.path.join(fxui_dir, 'tests', test)
                            for test in test_types[testtype]['default_tests']]
 
     kwargs['logger'] = commandline.setup_logging('Firefox UI - {} Tests'.format(testtype),
                                                  {"mach": sys.stdout})
 
+    kwargs['topsrcdir'] = topsrcdir
+    kwargs['topobjdir'] = topobjdir
+
     args = Namespace()
 
     for k, v in kwargs.iteritems():
         setattr(args, k, v)
 
     parser.verify_usage(args)
 
     failed = test_types[testtype]['cli_module'].cli(args=vars(args))
@@ -102,19 +105,21 @@ class MachCommands(MachCommandBase):
     @Command('firefox-ui-functional', category='testing',
              conditions=[conditions.is_firefox],
              description='Run the functional test suite of Firefox UI tests.',
              parser=setup_argument_parser_functional,
              )
     def run_firefox_ui_functional(self, **kwargs):
         kwargs['binary'] = kwargs['binary'] or self.get_binary_path('app')
         return run_firefox_ui_test(testtype='functional',
-                                   topsrcdir=self.topsrcdir, **kwargs)
+                                   topsrcdir=self.topsrcdir,
+                                   topobjdir=self.topobjdir, **kwargs)
 
     @Command('firefox-ui-update', category='testing',
              conditions=[conditions.is_firefox],
              description='Run the update test suite of Firefox UI tests.',
              parser=setup_argument_parser_update,
              )
     def run_firefox_ui_update(self, **kwargs):
         kwargs['binary'] = kwargs['binary'] or self.get_binary_path('app')
         return run_firefox_ui_test(testtype='update',
-                                   topsrcdir=self.topsrcdir, **kwargs)
+                                   topsrcdir=self.topsrcdir,
+                                   topobjdir=self.topobjdir, **kwargs)

--- a/testing/marionette/client/marionette_driver/geckoinstance.py
+++ b/testing/marionette/client/marionette_driver/geckoinstance.py
@@ -109,17 +109,18 @@ class GeckoInstance(object):
         "toolkit.telemetry.server": "https://%(server)s/dummy/telemetry/",
 
         # Enabling the support for File object creation in the content process.
         "dom.file.createInChild": True,
     }
 
     def __init__(self, host=None, port=None, bin=None, profile=None, addons=None,
                  app_args=None, symbols_path=None, gecko_log=None, prefs=None,
-                 workspace=None, verbose=0, headless=False):
+                 workspace=None, verbose=0, headless=False, topsrcdir=None,
+                 topobjdir=None):
         self.runner_class = Runner
         self.app_args = app_args or []
         self.runner = None
         self.symbols_path = symbols_path
         self.binary = bin
 
         self.marionette_host = host
         self.marionette_port = port
@@ -136,16 +137,18 @@ class GeckoInstance(object):
         self.required_prefs = deepcopy(self.required_prefs)
         if prefs:
             self.required_prefs.update(prefs)
 
         self._gecko_log_option = gecko_log
         self._gecko_log = None
         self.verbose = verbose
         self.headless = headless
+        self.topsrcdir = topsrcdir
+        self.topobjdir = topobjdir
 
     @property
     def gecko_log(self):
         if self._gecko_log:
             return self._gecko_log
 
         path = self._gecko_log_option
         if path != "-":
@@ -230,16 +233,21 @@ class GeckoInstance(object):
             process_args["logfile"] = self.gecko_log
 
         env = os.environ.copy()
 
         if self.headless:
             env["MOZ_HEADLESS"] = "1"
             env["DISPLAY"] = "77"  # Set a fake display.
 
+        if self.topsrcdir is not None:
+            env["MOZ_DEVELOPER_REPO_DIR"] = self.topsrcdir
+        if self.topobjdir is not None:
+            env["MOZ_DEVELOPER_OBJ_DIR"] = self.topobjdir
+
         # environment variables needed for crashreporting
         # https://developer.mozilla.org/docs/Environment_variables_affecting_crash_reporting
         env.update({"MOZ_CRASHREPORTER": "1",
                     "MOZ_CRASHREPORTER_NO_REPORT": "1",
                     "MOZ_CRASHREPORTER_SHUTDOWN": "1",
                     })
 
         return {

--- a/testing/marionette/harness/marionette_harness/runner/base.py
+++ b/testing/marionette/harness/marionette_harness/runner/base.py
@@ -514,17 +514,18 @@ class BaseMarionetteTestRunner(object):
                  testvars=None,
                  symbols_path=None,
                  shuffle=False, shuffle_seed=random.randint(0, sys.maxint), this_chunk=1,
                  total_chunks=1,
                  server_root=None, gecko_log=None, result_callbacks=None,
                  prefs=None, test_tags=None,
                  socket_timeout=BaseMarionetteArguments.socket_timeout_default,
                  startup_timeout=None, addons=None, workspace=None,
-                 verbose=0, e10s=True, emulator=False, headless=False, **kwargs):
+                 verbose=0, e10s=True, emulator=False, headless=False,
+                 topsrcdir=None, topobjdir=None, **kwargs):
         self._appName = None
         self._capabilities = None
         self._filename_pattern = None
         self._version_info = {}
 
         self.fixture_servers = {}
         self.fixtures = Fixtures()
         self.extra_kwargs = kwargs
@@ -556,16 +557,18 @@ class BaseMarionetteTestRunner(object):
         self.test_tags = test_tags
         self.startup_timeout = startup_timeout
         self.workspace = workspace
         # If no workspace is set, default location for gecko.log is .
         # and default location for profile is TMP
         self.workspace_path = workspace or os.getcwd()
         self.verbose = verbose
         self.headless = headless
+        self.topsrcdir = topsrcdir
+        self.topobjdir = topobjdir
 
         # self.e10s stores the desired configuration, whereas
         # self._e10s_from_browser is the cached value from querying e10s
         # in self.is_e10s
         self.e10s = e10s
         self._e10s_from_browser = None
         if self.e10s:
             self.prefs.update({
@@ -720,16 +723,18 @@ class BaseMarionetteTestRunner(object):
             os.mkdir(self.logdir)
 
         kwargs = {
             'socket_timeout': self.socket_timeout,
             'prefs': self.prefs,
             'startup_timeout': self.startup_timeout,
             'verbose': self.verbose,
             'symbols_path': self.symbols_path,
+            'topsrcdir': self.topsrcdir,
+            'topobjdir': self.topobjdir,
         }
         if self.bin or self.emulator:
             kwargs.update({
                 'host': 'localhost',
                 'port': 2828,
                 'app': self.app,
                 'app_args': self.app_args,
                 'profile': self.profile,

--- a/testing/marionette/mach_commands.py
+++ b/testing/marionette/mach_commands.py
@@ -25,30 +25,33 @@ def is_firefox_or_android(cls):
 
 def setup_marionette_argument_parser():
     from marionette_harness.runtests import MarionetteArguments
     from mozlog.structured import commandline
     parser = MarionetteArguments()
     commandline.add_logging_group(parser)
     return parser
 
-def run_marionette(tests, binary=None, topsrcdir=None, **kwargs):
+def run_marionette(tests, binary=None, topsrcdir=None, topobjdir=None, **kwargs):
     from mozlog.structured import commandline
 
     from marionette_harness.runtests import (
         MarionetteTestRunner,
         MarionetteHarness
     )
 
     parser = setup_marionette_argument_parser()
 
     if not tests:
         tests = [os.path.join(topsrcdir,
                  'testing/marionette/harness/marionette_harness/tests/unit-tests.ini')]
 
+    kwargs['topsrcdir'] = topsrcdir
+    kwargs['topobjdir'] = topobjdir
+
     args = argparse.Namespace(tests=tests)
 
     args.binary = binary
 
     for k, v in kwargs.iteritems():
         setattr(args, k, v)
 
     parser.verify_usage(args)
@@ -73,9 +76,10 @@ class MachCommands(MachCommandBase):
         if 'test_objects' in kwargs:
             tests = []
             for obj in kwargs['test_objects']:
                 tests.append(obj['file_relpath'])
             del kwargs['test_objects']
 
         if not kwargs.get('binary') and conditions.is_firefox(self):
             kwargs['binary'] = self.get_binary_path('app')
-        return run_marionette(tests, topsrcdir=self.topsrcdir, **kwargs)
+        return run_marionette(tests, topsrcdir=self.topsrcdir,
+                              topobjdir=self.topobjdir, **kwargs)