--- a/testing/web-platform/harness/wptrunner/browsers/firefox.py
+++ b/testing/web-platform/harness/wptrunner/browsers/firefox.py
@@ -168,18 +168,17 @@ class FirefoxBrowser(Browser):
         env["MOZ_DISABLE_NONLOCAL_CONNECTIONS"] = "1"
 
         locations = ServerLocations(filename=os.path.join(here, "server-locations.txt"))
 
         preferences = self.load_prefs()
 
         self.profile = FirefoxProfile(locations=locations,
                                       preferences=preferences)
-        self.profile.set_preferences({"marionette.enabled": True,
-                                      "marionette.port": self.marionette_port,
+        self.profile.set_preferences({"marionette.port": self.marionette_port,
                                       "dom.disable_open_during_load": False,
                                       "network.dns.localDomains": ",".join(hostnames),
                                       "network.proxy.type": 0,
                                       "places.history.enabled": False})
         if self.e10s:
             self.profile.set_preferences({"browser.tabs.remote.autostart": True})
 
         if self.leak_check and kwargs.get("check_leaks", True):