diff --git a/testing/mozharness/scripts/web_platform_tests.py b/testing/mozharness/scripts/web_platform_tests.py
index 6c86bf299c82..ed813cbe8f19 100755
--- a/testing/mozharness/scripts/web_platform_tests.py
+++ b/testing/mozharness/scripts/web_platform_tests.py
@@ -29,6 +29,7 @@ class WebPlatformTest(TestingMixin, MercurialScript, BlobUploadMixin):
         [['--e10s'], {
             "action": "store_true",
             "dest": "e10s",
+            "default": False,
             "help": "Run with e10s enabled"}
          ],
         [["--total-chunks"], {
@@ -135,8 +136,8 @@ class WebPlatformTest(TestingMixin, MercurialScript, BlobUploadMixin):
         for test_type in c.get("test_type", []):
             cmd.append("--test-type=%s" % test_type)
 
-        if c.get("e10s"):
-            cmd.append("--e10s")
+        if not c["e10s"]:
+            cmd.append("--disable-e10s")
 
         for opt in ["total_chunks", "this_chunk"]:
             val = c.get(opt)
diff --git a/testing/web-platform/harness/wptrunner/wptcommandline.py b/testing/web-platform/harness/wptrunner/wptcommandline.py
index d8b29fddc668..cafd6863e7a8 100644
--- a/testing/web-platform/harness/wptrunner/wptcommandline.py
+++ b/testing/web-platform/harness/wptrunner/wptcommandline.py
@@ -159,8 +159,8 @@ def create_parser(product_choices=None):
     gecko_group = parser.add_argument_group("Gecko-specific")
     gecko_group.add_argument("--prefs-root", dest="prefs_root", action="store", type=abs_path,
                              help="Path to the folder containing browser prefs")
-    gecko_group.add_argument("--e10s", dest="gecko_e10s", action="store_true",
-                             help="Run tests with electrolysis preferences")
+    gecko_group.add_argument("--disable-e10s", dest="gecko_e10s", action="store_false", default=True,
+                             help="Run tests without electrolysis preferences")
 
     b2g_group = parser.add_argument_group("B2G-specific")
     b2g_group.add_argument("--b2g-no-backup", action="store_true", default=False,