--- a/testing/mozharness/scripts/web_platform_tests.py
+++ b/testing/mozharness/scripts/web_platform_tests.py
@@ -142,17 +142,18 @@ class WebPlatformTest(TestingMixin, Merc
         cmd += ["--log-raw=-",
                 "--log-raw=%s" % os.path.join(dirs["abs_blob_upload_dir"],
                                               "wpt_raw.log"),
                 "--log-errorsummary=%s" % os.path.join(dirs["abs_blob_upload_dir"],
                                                        "wpt_errorsummary.log"),
                 "--binary=%s" % self.binary_path,
                 "--symbols-path=%s" % self.query_symbols_url(),
                 "--stackwalk-binary=%s" % self.query_minidump_stackwalk(),
-                "--stackfix-dir=%s" % os.path.join(dirs["abs_test_install_dir"], "bin")]
+                "--stackfix-dir=%s" % os.path.join(dirs["abs_test_install_dir"], "bin"),
+                "--run-by-dir=3"]
 
         for test_type in c.get("test_type", []):
             cmd.append("--test-type=%s" % test_type)
 
         if not c["e10s"]:
             cmd.append("--disable-e10s")
 
         for opt in ["total_chunks", "this_chunk"]: