--- a/testing/marionette/harness/marionette_harness/runner/base.py
+++ b/testing/marionette/harness/marionette_harness/runner/base.py
@@ -548,22 +548,27 @@ 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.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({
                 'browser.tabs.remote.autostart': True,
                 'browser.tabs.remote.force-enable': True,
-                'extensions.e10sBlocksEnabling': False
+                'extensions.e10sBlocksEnabling': False,
             })
 
         def gather_debug(test, status):
             # No screenshots and page source for skipped tests
             if status == "SKIP":
                 return
 
             rv = {}
@@ -796,16 +801,31 @@ class BaseMarionetteTestRunner(object):
         for test in self.manifest_skipped_tests:
             name = os.path.basename(test['path'])
             self.logger.test_start(name)
             self.logger.test_end(name,
                                  'SKIP',
                                  message=test['disabled'])
             self.todo += 1
 
+    @property
+    def is_e10s(self):
+        """Query the browser on whether E10s (Electrolysis) is enabled."""
+        if self._e10s_from_browser is not None:
+            return self._e10s_from_browser
+
+        if self.marionette is None or self.marionette.session is None:
+            raise Exception("No Marionette session to query e10s state")
+
+        with self.marionette.using_context("chrome"):
+            self._e10s_from_browser = self.marionette.execute_script(
+                "return Services.appinfo.browserTabsRemoteAutostart")
+
+        return self.is_e10s
+
     def run_tests(self, tests):
         start_time = time.time()
         self._initialize_test_run(tests)
 
         if self.marionette is None:
             self.marionette = self.driverclass(**self._build_kwargs())
             self.logger.info("Profile path is %s" % self.marionette.profile_path)
 
@@ -824,27 +844,26 @@ class BaseMarionetteTestRunner(object):
         device_info = None
         if self.marionette.instance and self.emulator:
             try:
                 device_info = self.marionette.instance.runner.device.dm.getInfo()
             except Exception:
                 self.logger.warning('Could not get device info', exc_info=True)
 
         self.marionette.start_session()
-        with self.marionette.using_context("chrome"):
-            appinfo_e10s = self.marionette.execute_script(
-                "return Services.appinfo.browserTabsRemoteAutostart")
+
         self.marionette.delete_session()
 
-        self.logger.info("e10s is {}".format("enabled" if appinfo_e10s else "disabled"))
-        if self.e10s != appinfo_e10s:
-            message_e10s = ("BaseMarionetteTestRunner configuration (self.e10s) does "
-                            "not match browser appinfo")
+        self.logger.info("e10s is {}".format("enabled" if self.is_e10s else "disabled"))
+        print "self.is_e10s=%s" % self.is_e10s
+        print "self.e10s=%s" % self.e10s
+        if self.e10s != self.is_e10s:
             self.cleanup()
-            raise AssertionError(message_e10s)
+            raise AssertionError("BaseMarionetteTestRunner configuration (self.e10s) "
+                "does not match browser appinfo (self.is_e10s)")
 
         tests_by_group = defaultdict(list)
         for test in self.tests:
             tests_by_group[test['group']].append(test['filepath'])
 
         self.logger.suite_start(tests_by_group,
                                 version_info=self.version_info,
                                 device_info=device_info)

--- a/testing/marionette/harness/marionette_harness/tests/harness_unit/test_marionette_runner.py
+++ b/testing/marionette/harness/marionette_harness/tests/harness_unit/test_marionette_runner.py
@@ -1,17 +1,18 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import manifestparser
 import pytest
 
-from mock import Mock, patch, mock_open, sentinel, DEFAULT
+from mock import MagicMock, Mock, patch, mock_open, sentinel, DEFAULT
 
+from marionette_driver.marionette import Marionette
 from marionette_harness.runtests import MarionetteTestRunner
 
 
 @pytest.fixture
 def runner(mach_parsed_kwargs):
     """
     MarionetteTestRunner instance initialized with default options.
     """
@@ -25,17 +26,17 @@ def mock_runner(runner, mock_marionette,
     self.marionette and other properties,
     to enable testing runner.run_tests().
     """
     runner.driverclass = mock_marionette
     for attr in ['run_test_set', '_capabilities']:
         setattr(runner, attr, Mock())
     runner._appName = 'fake_app'
     # simulate that browser runs with e10s by default
-    runner._appinfo = {'browserTabsRemoteAutostart': True}
+    runner._e10s_from_browser = True
     monkeypatch.setattr('marionette_harness.runner.base.mozversion', Mock())
     return runner
 
 
 @pytest.fixture
 def build_kwargs_using(mach_parsed_kwargs):
     '''Helper function for test_build_kwargs_* functions'''
     def kwarg_builder(new_items, return_socket=False):
@@ -427,17 +428,18 @@ def test_e10s_option_sets_prefs(mach_par
         'extensions.e10sBlocksEnabling': False
     }
     for k,v in e10s_prefs.iteritems():
         if k == 'extensions.e10sBlocksEnabling' and not e10s:
             continue
         assert runner.prefs.get(k, False) == (v and e10s)
 
 def test_e10s_option_clash_raises(mock_runner):
-    mock_runner.e10s = False
+    mock_runner._e10s_from_browser = False
+
     with pytest.raises(AssertionError) as e:
         mock_runner.run_tests([u'test_fake_thing.py'])
         assert "configuration (self.e10s) does not match browser appinfo" in e.value.message
 
 if __name__ == '__main__':
     import sys
     sys.exit(pytest.main(
         ['--log-tbpl=-', __file__]))