--- a/testing/web-platform/mach_commands.py
+++ b/testing/web-platform/mach_commands.py
@@ -3,18 +3,20 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 # Integrates the web-platform-tests test runner with mach.
 
 from __future__ import absolute_import, unicode_literals, print_function
 
 import os
 import sys
+import textwrap
 
 from mozbuild.base import (
+    BuildEnvironmentNotFoundException,
     MachCommandBase,
     MachCommandConditions as conditions,
     MozbuildObject,
 )
 
 from mach.decorators import (
     CommandProvider,
     Command,
@@ -27,42 +29,56 @@ class InvalidTestPathError(Exception):
 
 class WebPlatformTestsRunner(MozbuildObject):
     """Run web platform tests."""
 
     def setup_kwargs(self, kwargs):
         from wptrunner import wptcommandline
 
         build_path = os.path.join(self.topobjdir, 'build')
-        if build_path not in sys.path:
-            sys.path.append(build_path)
+        if os.path.exists(build_path):
+            have_objdir_build = True
+            if build_path not in sys.path:
+                sys.path.append(build_path)
+        else:
+            have_objdir_build = False
+
+        if have_objdir_build:
+            if kwargs["binary"] is None:
+                kwargs["binary"] = self.get_binary_path('app')
+
+            if kwargs["certutil_binary"] is None:
+                kwargs["certutil_binary"] = self.get_binary_path('certutil')
+        else:
+            if kwargs["binary"] is None or kwargs["certutil_binary"] is None:
+                print(textwrap.fill("No build found. Either run |mach build|, "
+                                    "or provide the path to a binary and "
+                                    "certutil explicitly using --binary and "
+                                    "--certutil-binary"))
+                sys.exit(1)
+
+        src_base = os.path.join(self.topsrcdir, 'testing', 'web-platform')
 
         if kwargs["config"] is None:
-            kwargs["config"] = os.path.join(self.topsrcdir, 'testing', 'web-platform', 'wptrunner.ini')
-
-        if kwargs["binary"] is None:
-            kwargs["binary"] = self.get_binary_path('app')
+            kwargs["config"] = os.path.join(src_base, 'wptrunner.ini')
 
         if kwargs["prefs_root"] is None:
-            kwargs["prefs_root"] = os.path.join(self.topobjdir, '_tests', 'web-platform', "prefs")
-
-        if kwargs["certutil_binary"] is None:
-            kwargs["certutil_binary"] = self.get_binary_path('certutil')
+            kwargs["prefs_root"] = os.path.join(self.topsrcdir, 'testing', 'profiles')
 
         here = os.path.split(__file__)[0]
 
         if kwargs["ssl_type"] in (None, "pregenerated"):
             if kwargs["ca_cert_path"] is None:
-                kwargs["ca_cert_path"] = os.path.join(here, "certs", "cacert.pem")
+                kwargs["ca_cert_path"] = os.path.join(src_base, "certs", "cacert.pem")
 
             if kwargs["host_key_path"] is None:
-                kwargs["host_key_path"] = os.path.join(here, "certs", "web-platform.test.key")
+                kwargs["host_key_path"] = os.path.join(src_base, "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["host_cert_path"] = os.path.join(src_base, "certs", "web-platform.test.pem")
 
         kwargs["capture_stdio"] = True
 
         kwargs = wptcommandline.check_args(kwargs)
 
     def run_tests(self, **kwargs):
         from wptrunner import wptrunner
 
@@ -288,17 +304,16 @@ def create_parser_manifest_update():
 
 @CommandProvider
 class MachCommands(MachCommandBase):
     def setup(self):
         self._activate_virtualenv()
 
     @Command("web-platform-tests",
              category="testing",
-             conditions=[conditions.is_firefox],
              parser=create_parser_wpt)
     def run_web_platform_tests(self, **params):
         self.setup()
 
         if "test_objects" in params:
             for item in params["test_objects"]:
                 params["include"].append(item["name"])
             del params["test_objects"]
@@ -307,17 +322,16 @@ class MachCommands(MachCommandBase):
 
         if params["list_test_groups"]:
             return wpt_runner.list_test_groups(**params)
         else:
             return wpt_runner.run_tests(**params)
 
     @Command("wpt",
              category="testing",
-             conditions=[conditions.is_firefox],
              parser=create_parser_wpt)
     def run_wpt(self, **params):
         return self.run_web_platform_tests(**params)
 
     @Command("web-platform-tests-update",
              category="testing",
              parser=create_parser_update)
     def update_web_platform_tests(self, **params):