--- a/testing/tps/tps/testrunner.py
+++ b/testing/tps/tps/testrunner.py
@@ -366,16 +366,19 @@ class TPSTestRunner(object):
         # correct authentication type. Without this pref set to a value
         # without an '@' character, Sync will initialize for FxA.
         if self.config.get('auth_type', 'fx_account') != "fx_account":
             self.preferences.update({'services.sync.username': "dummy"})
 
         if self.debug:
             self.preferences.update(self.debug_preferences)
 
+        if 'preferences' in self.config:
+            self.preferences.update(self.config['preferences'])
+
     def run_tests(self):
         # delete the logfile if it already exists
         if os.access(self.logfile, os.F_OK):
             os.remove(self.logfile)
 
         # Copy the system env variables, and update them for custom settings
         self.env = os.environ.copy()
         self.env.update(self.extra_env)