--- a/testing/web-platform/mach_commands.py
+++ b/testing/web-platform/mach_commands.py
@@ -60,16 +60,19 @@ class WebPlatformTestsRunner(MozbuildObj
             if kwargs["host_key_path"] is None:
                 kwargs["host_key_path"] = os.path.join(here, "certs", "web-platform.test.key")
 
             if kwargs["host_cert_path"] is None:
                 kwargs["host_cert_path"] = os.path.join(here, "certs", "web-platform.test.pem")
 
         kwargs["capture_stdio"] = True
 
+        if kwargs["webdriver_binary"] is None:
+            kwargs["webdriver_binary"] = self.get_binary_path("geckodriver")
+
         kwargs = wptcommandline.check_args(kwargs)
 
     def setup_kwargs_wptrun(self, kwargs):
         from wptrunner import wptcommandline
         here = os.path.join(self.topsrcdir, 'testing', 'web-platform')
 
         sys.path.insert(0, os.path.join(here, "tests", "tools"))