fix(firefox_web_driver): configure Firefox's locale at fixture time

Otherwise the fixture isn't set up to request the language expected by
the tests that use it.

securedrop/tests/functional/conftest.py

@@ -17,14 +17,18 @@
 from source_user import SourceUser
 from tests.factories import SecureDropConfigFactory
 from tests.functional.db_session import get_database_session
+from tests.functional.pageslayout.utils import list_locales
 from tests.functional.web_drivers import WebDriverTypeEnum, get_web_driver
 from tests.utils.i18n import get_test_locales
 
 
 # Function-scoped so that tests can be run in parallel if needed
 @pytest.fixture
 def firefox_web_driver() -> WebDriver:  # type: ignore
-    with get_web_driver(web_driver_type=WebDriverTypeEnum.FIREFOX) as web_driver:
+    with get_web_driver(
+        web_driver_type=WebDriverTypeEnum.FIREFOX,
+        accept_languages=",".join(list_locales()).replace("_", "-"),
+    ) as web_driver:
         yield web_driver