--- a/testing/mozharness/mozharness/mozilla/testing/firefox_ui_tests.py
+++ b/testing/mozharness/mozharness/mozilla/testing/firefox_ui_tests.py
@@ -211,16 +211,19 @@ class FirefoxUITests(TestingMixin, VCSTo
 
             # logging options
             '--gecko-log=-',  # output from the gecko process redirected to stdout
             '--log-raw=-',  # structured log for output parser redirected to stdout
 
             # additional reports helpful for Jenkins and inpection via Treeherder
             '--log-html', os.path.join(dirs['abs_blob_upload_dir'], 'report.html'),
             '--log-xunit', os.path.join(dirs['abs_blob_upload_dir'], 'report.xml'),
+
+            # Enable tracing output to log transmission protocol
+            '-vv',
         ]
 
         # Collect all pass-through harness options to the script
         cmd.extend(self.query_harness_args())
 
         # Translate deprecated --e10s flag
         if not self.config.get('e10s'):
             cmd.append('--disable-e10s')