diff --git a/.travis.yml b/.travis.yml
index 7e7461e6..70b3b03d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,7 +9,7 @@ _aliases:
       paths:
         - results
   - &nightly
-    env: FIREFOX=nightly
+    env: FIREFOX=nightly PYTEST_ADDOPTS=-n=8
     addons:
       <<: *artifacts
       firefox: latest-nightly
diff --git a/Pipfile b/Pipfile
index 13253905..0a0786f4 100644
--- a/Pipfile
+++ b/Pipfile
@@ -1,17 +1,12 @@
 [[source]]
-
 url = "https://pypi.python.org/simple"
 verify_ssl = true
 name = "pypi"
 
-
 [packages]
-
 selenium = "==3.11.0"
 
-
 [dev-packages]
-
 coveralls = "==1.3.0"
 flake8 = "==3.5.0"
 flake8-docstrings = "==1.3.0"
@@ -19,3 +14,4 @@ flake8-isort = "==2.5"
 pytest = "==3.5.0"
 pytest-cov = "==2.5.1"
 pytest-selenium = "==1.12.0"
+pytest-xdist = "==1.22.2"
diff --git a/tests/webserver.py b/tests/webserver.py
index 130d52f8..b1fc5e4e 100644
--- a/tests/webserver.py
+++ b/tests/webserver.py
@@ -30,7 +30,7 @@ def translate_path(self, path):
 class WebServer(object):
     """Web server for serving local files within the /web directory."""
 
-    def __init__(self, host='', port=8000):
+    def __init__(self, host='', port=0):
         """Set up web server.
 
         Args:
diff --git a/tox.ini b/tox.ini
index 74dfee02..36468a33 100644
--- a/tox.ini
+++ b/tox.ini
@@ -3,7 +3,7 @@ envlist = py{27,36}, flake8, docs
 
 [testenv]
 recreate = True
-passenv = DISPLAY MOZ_HEADLESS TRAVIS TRAVIS_JOB_ID TRAVIS_BRANCH
+passenv = DISPLAY MOZ_HEADLESS PYTEST_ADDOPTS TRAVIS TRAVIS_JOB_ID TRAVIS_BRANCH
 deps = pipenv
 commands =
     pipenv install --dev --skip-lock