diff --git a/testing/test_firefox.py b/testing/test_firefox.py
index 2e3ab7aa..86284065 100644
--- a/testing/test_firefox.py
+++ b/testing/test_firefox.py
@@ -103,6 +103,7 @@ def test_preferences(base_url, selenium):
     )
 
 
+@pytest.mark.xfail(reason="https://github.com/SeleniumHQ/selenium/pull/5069")
 def test_extension(testdir):
     """Test that a firefox extension can be added when starting Firefox."""
     import os
diff --git a/tox.ini b/tox.ini
index 3b1f8d45..fb8669e2 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,8 +9,10 @@ envlist = py{27,36,37,py,py3}, docs, flake8
 [testenv]
 passenv = DISPLAY PYTEST_ADDOPTS
 setenv = MOZ_HEADLESS=1
-deps = pytest-localserver
-commands = pytest -v -r a {posargs}
+deps =
+    pytest-localserver
+    pytest-xdist
+commands = pytest -n auto -v -r a {posargs}
 
 [testenv:docs]
 basepython = python