--- a/testing/web-platform/tests/tools/wpt/tests/test_wpt.py
+++ b/testing/web-platform/tests/tools/wpt/tests/test_wpt.py
@@ -164,33 +164,35 @@ def test_list_tests_invalid_manifest(man
 @pytest.mark.xfail(sys.platform == "win32",
                    reason="Tests currently don't work on Windows for path reasons")
 def test_run_firefox(manifest_dir):
     # TODO: It seems like there's a bug in argparse that makes this argument order required
     # should try to work around that
     if is_port_8000_in_use():
         pytest.skip("port 8000 already in use")
 
-    os.environ["MOZ_HEADLESS"] = "1"
-    try:
-        if sys.platform == "darwin":
-            fx_path = os.path.join(wpt.localpaths.repo_root, "_venv", "browsers", "nightly", "Firefox Nightly.app")
-        else:
-            fx_path = os.path.join(wpt.localpaths.repo_root, "_venv", "browsers", "nightly", "firefox")
-        if os.path.exists(fx_path):
-            shutil.rmtree(fx_path)
-        with pytest.raises(SystemExit) as excinfo:
-            wpt.main(argv=["run", "--no-pause", "--install-browser", "--yes",
-                           "--metadata", manifest_dir,
-                           "firefox", "/dom/nodes/Element-tagName.html"])
-        assert os.path.exists(fx_path)
+    if sys.platform == "darwin":
+        fx_path = os.path.join(wpt.localpaths.repo_root, "_venv", "browsers", "nightly", "Firefox Nightly.app")
+    else:
+        fx_path = os.path.join(wpt.localpaths.repo_root, "_venv", "browsers", "nightly", "firefox")
+    if os.path.exists(fx_path):
         shutil.rmtree(fx_path)
-        assert excinfo.value.code == 0
-    finally:
-        del os.environ["MOZ_HEADLESS"]
+    with pytest.raises(SystemExit) as excinfo:
+        wpt.main(argv=["run", "--no-pause", "--install-browser", "--yes",
+                       # The use of `--binary-args` is intentional: it
+                       # demonstrates that internally-managed command-line
+                       # arguments are properly merged with those specified by
+                       # the user. See
+                       # https://github.com/web-platform-tests/wpt/pull/13154
+                       "--binary-arg=-headless",
+                       "--metadata", manifest_dir,
+                       "firefox", "/dom/nodes/Element-tagName.html"])
+    assert os.path.exists(fx_path)
+    shutil.rmtree(fx_path)
+    assert excinfo.value.code == 0
 
 
 @pytest.mark.slow
 @pytest.mark.xfail(sys.platform == "win32",
                    reason="Tests currently don't work on Windows for path reasons")
 def test_run_chrome(manifest_dir):
     if is_port_8000_in_use():
         pytest.skip("port 8000 already in use")

--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/browsers/firefox.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/browsers/firefox.py
@@ -246,19 +246,21 @@ class FirefoxBrowser(Browser):
         # Bug 1262954: winxp + e10s, disable hwaccel
         if (self.e10s and platform.system() in ("Windows", "Microsoft") and
             '5.1' in platform.version()):
             self.profile.set_preferences({"layers.acceleration.disabled": True})
 
         if self.ca_certificate_path is not None:
             self.setup_ssl()
 
+        args = self.binary_args[:] if self.binary_args else []
+        args += [cmd_arg("marionette"), "about:blank"]
+
         debug_args, cmd = browser_command(self.binary,
-                                          self.binary_args if self.binary_args else [] +
-                                          [cmd_arg("marionette"), "about:blank"],
+                                          args,
                                           self.debug_info)
 
         self.runner = FirefoxRunner(profile=self.profile,
                                     binary=cmd[0],
                                     cmdargs=cmd[1:],
                                     env=env,
                                     process_class=ProcessHandler,
                                     process_args={"processOutputLine": [self.on_output]})