--- a/testing/mozharness/mozharness/mozilla/testing/testbase.py
+++ b/testing/mozharness/mozharness/mozilla/testing/testbase.py
@@ -4,28 +4,30 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this file,
 # You can obtain one at http://mozilla.org/MPL/2.0/.
 # ***** END LICENSE BLOCK *****
 
 import copy
 import os
 import platform
 import pprint
-import re
 import urllib2
 import json
 import socket
 
 from mozharness.base.errors import BaseErrorList
 from mozharness.base.log import FATAL, WARNING
 from mozharness.base.python import (
     ResourceMonitoringMixin,
     VirtualenvMixin,
     virtualenv_config_options,
 )
+from mozharness.base.vcs.vcsbase import (
+    VCSMixin,
+)
 from mozharness.mozilla.buildbot import BuildbotMixin, TBPL_WARNING
 from mozharness.mozilla.proxxy import Proxxy
 from mozharness.mozilla.structuredlog import StructuredOutputParser
 from mozharness.mozilla.taskcluster_helper import TaskClusterArtifactFinderMixin
 from mozharness.mozilla.testing.unittest import DesktopUnittestOutputParser
 from mozharness.mozilla.testing.try_tools import TryToolsMixin, try_config_options
 from mozharness.mozilla.tooltool import TooltoolMixin
 
@@ -97,17 +99,18 @@ testing_config_options = [
      "choices": ['ondemand', 'true'],
      "help": "Download and extract crash reporter symbols.",
       }],
 ] + copy.deepcopy(virtualenv_config_options) + copy.deepcopy(try_config_options)
 
 
 # TestingMixin {{{1
 class TestingMixin(VirtualenvMixin, BuildbotMixin, ResourceMonitoringMixin,
-                   TaskClusterArtifactFinderMixin, TooltoolMixin, TryToolsMixin):
+                   TaskClusterArtifactFinderMixin, TooltoolMixin, TryToolsMixin,
+                   VCSMixin):
     """
     The steps to identify + download the proper bits for [browser] unit
     tests and Talos.
     """
 
     installer_url = None
     installer_path = None
     binary_path = None
@@ -115,16 +118,65 @@ class TestingMixin(VirtualenvMixin, Buil
     test_packages_url = None
     symbols_url = None
     symbols_path = None
     jsshell_url = None
     minidump_stackwalk_path = None
     default_tools_repo = 'https://hg.mozilla.org/build/tools'
     proxxy = None
 
+    def query_abs_dirs(self):
+        dirs = super(TestingMixin, self).query_abs_dirs()
+
+        if self.topsrcdir:
+            dirs['checkout'] = self.topsrcdir
+        else:
+            dirs['checkout'] = os.path.join(dirs['base_work_dir'], 'checkout')
+
+        if 'HG_SHARE_BASE_DIR' in os.environ:
+            dirs['hg_shared'] = os.environ['HG_SHARE_BASE_DIR']
+        else:
+            dirs['hg_shared'] = os.path.join(dirs['base_work_dir'], 'hg-shared')
+
+        return dirs
+
+    def ensure_firefox_checkout(self):
+        """Action that ensures we have a source checkout available.
+
+        If a source checkout is not available, we create one.
+        """
+        if self.topsrcdir:
+            self.info('already running from a source checkout; nothing to do')
+            return
+
+        self.info('not running from source checkout')
+
+        bb_config = getattr(self, 'buildbot_config', None)
+        if not bb_config:
+            self.fatal('buildbot config not loaded; unable to proceed '
+                       '(a buildbot config must be loaded to obtain VCS info)')
+
+        bb_props = bb_config['properties']
+
+        dirs = self.query_abs_dirs()
+
+        args = {
+            'repo': 'https://hg.mozilla.org/%s' % bb_props['repo_path'],
+            'dest': dirs['checkout'],
+            'vcs_share_base': dirs['hg_shared'],
+            'revision': bb_props['revision'],
+            'clone_with_purge': True,
+            # Always use the unified repo as the upstream because it is
+            # stored more efficiently.
+            'clone_upstream_url': 'https://hg.mozilla.org/mozilla-unified',
+        }
+
+        self.vcs_checkout(vcs='hg', **args)
+        self.topsrcdir = dirs['checkout']
+
     def _query_proxxy(self):
         """manages the proxxy"""
         if not self.proxxy:
             self.proxxy = Proxxy(self.config, self.log_obj)
         return self.proxxy
 
     def download_proxied_file(self, url, file_name=None, parent_dir=None,
                               create_parent_dir=True, error_level=FATAL,

--- a/testing/mozharness/scripts/web_platform_tests.py
+++ b/testing/mozharness/scripts/web_platform_tests.py
@@ -53,16 +53,17 @@ class WebPlatformTest(TestingMixin, Merc
         copy.deepcopy(blobupload_config_options)
 
     def __init__(self, require_config_file=True):
         super(WebPlatformTest, self).__init__(
             config_options=self.config_options,
             all_actions=[
                 'clobber',
                 'read-buildbot-config',
+                'ensure-firefox-checkout',
                 'download-and-extract',
                 'fetch-geckodriver',
                 'create-virtualenv',
                 'pull',
                 'install',
                 'run-tests',
             ],
             require_config_file=require_config_file,