--- a/testing/marionette/harness/marionette/runtests.py
+++ b/testing/marionette/harness/marionette/runtests.py
@@ -26,19 +26,21 @@ class MarionetteArguments(BaseMarionette
         BaseMarionetteArguments.__init__(self, **kwargs)
         self.register_argument_container(BrowserMobProxyArguments())
 
 
 class MarionetteHarness(object):
     def __init__(self,
                  runner_class=MarionetteTestRunner,
                  parser_class=MarionetteArguments,
+                 testcase_class=MarionetteTestCase,
                  args=None):
         self._runner_class = runner_class
         self._parser_class = parser_class
+        self._testcase_class = testcase_class
         self.args = args or self.parse_args()
 
     def parse_args(self, logger_defaults=None):
         parser = self._parser_class(usage='%(prog)s [options] test_file_or_dir <test_file_or_dir> ...')
         parser.add_argument('--version', action='version',
             help="Show version information.",
             version="%(prog)s {version}"
                     " (using marionette-driver: {driver_version}, ".format(
@@ -52,17 +54,17 @@ class MarionetteHarness(object):
         logger = mozlog.commandline.setup_logging(
             args.logger_name, args, logger_defaults or {"tbpl": sys.stdout})
 
         args.logger = logger
         return vars(args)
 
     def process_args(self):
         if self.args.get('pydebugger'):
-            MarionetteTestCase.pydebugger = __import__(self.args['pydebugger'])
+            self._testcase_class.pydebugger = __import__(self.args['pydebugger'])
 
     def run(self):
         try:
             self.process_args()
             tests = self.args.pop('tests')
             runner = self._runner_class(**self.args)
             runner.run_tests(tests)
             return runner.failed + runner.crashed
@@ -70,28 +72,28 @@ class MarionetteHarness(object):
             logger = self.args.get('logger')
             if logger:
                 logger.error('Failure during test execution.',
                                        exc_info=True)
             raise
 
 
 def cli(runner_class=MarionetteTestRunner, parser_class=MarionetteArguments,
-        harness_class=MarionetteHarness, args=None):
+        harness_class=MarionetteHarness, testcase_class=MarionetteTestCase, args=None):
     """
     Call the harness to parse args and run tests.
 
     The following exit codes are expected:
     - Test failures: 10
     - Harness/other failures: 1
     - Success: 0
     """
     logger = mozlog.commandline.setup_logging('Marionette test runner', {})
     try:
-        failed = harness_class(runner_class, parser_class, args=args).run()
+        failed = harness_class(runner_class, parser_class, testcase_class, args=args).run()
         if failed > 0:
             sys.exit(10)
     except Exception:
         logger.error('Failure during harness setup', exc_info=True)
         sys.exit(1)
     sys.exit(0)
 
 if __name__ == "__main__":