--- a/testing/firefox-ui/harness/firefox_ui_harness/runners/base.py
+++ b/testing/firefox-ui/harness/firefox_ui_harness/runners/base.py
@@ -15,16 +15,27 @@ from marionette_harness import BaseMario
 class FirefoxUITestRunner(BaseMarionetteTestRunner):
 
     def __init__(self, **kwargs):
         super(FirefoxUITestRunner, self).__init__(**kwargs)
 
         # select the appropriate GeckoInstance
         self.app = 'fxdesktop'
 
+        # low-noise log messages useful in tests
+        # TODO: should be moved to individual tests once bug 1386810
+        # is fixed
+        moz_log = ''
+        if 'MOZ_LOG' in os.environ:
+            moz_log = os.environ['MOZ_LOG']
+        if len(moz_log) > 0:
+            moz_log += ','
+        moz_log += 'UrlClassifierStreamUpdater:1'
+        os.environ['MOZ_LOG'] = moz_log
+
         self.test_handlers = [MarionetteTestCase]
 
     def duplicate_application(self, application_folder):
         """Creates a copy of the specified binary."""
 
         if self.workspace:
             target_folder = os.path.join(self.workspace_path, 'application.copy')
         else: