--- a/layout/tools/reftest/runreftest.py
+++ b/layout/tools/reftest/runreftest.py
@@ -322,16 +322,20 @@ class RefTest(object):
         if platform.system() in ("Windows", "Microsoft") and \
            '6.1' in platform.version():
             prefs['reftest.nocache'] = True
 
         if options.marionette:
             port = options.marionette.split(':')[1]
             prefs['marionette.defaultPrefs.port'] = int(port)
 
+            # Enable tracing output for detailed failures in case of
+            # failing connection attempts, and hangs (bug 1397201)
+            prefs['marionette.logging'] = "TRACE"
+
         preference_file = os.path.join(here, 'reftest-preferences.js')
         prefs.update(mozprofile.Preferences.read_prefs(preference_file))
 
         for v in options.extraPrefs:
             thispref = v.split('=')
             if len(thispref) < 2:
                 print "Error: syntax error in --setpref=" + v
                 sys.exit(1)

--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -1800,16 +1800,22 @@ toolbar#nav-bar {
         options.extraPrefs.append(
             "dom.ipc.tabs.nested.enabled=%s" %
             ('true' if options.nested_oop else 'false'))
 
         options.extraPrefs.append(
             "idle.lastDailyNotification=%d" %
             int(time.time()))
 
+        # Enable tracing output for detailed failures in case of
+        # failing connection attempts, and hangs (bug 1397201)
+        options.extraPrefs.append(
+            "marionette.logging=%s" %
+            "TRACE")
+
         # get extensions to install
         extensions = self.getExtensionsToInstall(options)
 
         # preferences
         preferences = [
             os.path.join(
                 SCRIPT_DIR,
                 'profile_data',