--- a/testing/marionette/driver/marionette_driver/geckoinstance.py
+++ b/testing/marionette/driver/marionette_driver/geckoinstance.py
@@ -34,16 +34,19 @@ class GeckoInstance(object):
         "datareporting.healthreport.uploadEnabled": False,
         "datareporting.policy.dataSubmissionEnabled": False,
         "datareporting.policy.dataSubmissionPolicyAccepted": False,
         "dom.ipc.reportProcessHangs": False,
         "focusmanager.testmode": True,
         "marionette.defaultPrefs.enabled": True,
         "startup.homepage_welcome_url": "about:blank",
         "toolkit.telemetry.enabled": False,
+        # Until Bug 1238095 is fixed, we have to enable CPOWs in order
+        # for Marionette tests to work properly.
+        "dom.ipc.cpows.forbid-unsafe-from-browser": False,
     }
 
     def __init__(self, host, port, bin, profile=None, addons=None,
                  app_args=None, symbols_path=None, gecko_log=None, prefs=None,
                  workspace=None, verbose=0):
         self.marionette_host = host
         self.marionette_port = port
         self.bin = bin