This test fails on Firefox[1] but works fine on Chrome.

Steps to reproduce, from signed_in_and_approved_users.feature[2]:

Scenario: Using message group selector to choose a project to translate

Scenario: Using message group selector to choose a project to translate
  When I change my interface language away from English
    And I go to the main page
    And I click the message group selector tile
    And I click a message group
  Then I should see a list of messages in the the translation view

Additionally, opening Special:MainPage[3] in Firefox results in 404 and TypeError[4].

1: https://integration.wikimedia.org/ci/view/BrowserTests/job/browsertests-TwnMainPage-sandbox.translatewiki.net-linux-firefox/lastCompletedBuild/testReport/(root)/View%20for%20approved%20users/Using_message_group_selector_to_choose_a_project_to_translate/
2: https://github.com/wikimedia/mediawiki-extensions-TwnMainPage/blob/master/tests/browser/features/signed_in_and_approved_users.feature#L54-L59
3: http://sandbox.translatewiki.net/wiki/Special:MainPage
4:
GET http://sandbox.translatewiki.net/wiki/Special:MainPage [HTTP/1.1 200 OK 283ms]
GET http://sandbox.translatewiki.net/w/load.php [HTTP/1.1 304 Not Modified 96ms]
GET http://sandbox.translatewiki.net/w/load.php [HTTP/1.1 200 OK 1430ms]
GET https://secure.gravatar.com/avatar/296260a40bc83aee70792fc51e39dadc [HTTP/1.1 304 Not Modified 661ms]
GET http://sandbox.translatewiki.net/w/api.php [HTTP/1.1 200 OK 189ms]
Use of getPreventDefault() is deprecated. Use defaultPrevented instead. load.php:48
POST http://sandbox.translatewiki.net/w/webfiles/jserror.php [HTTP/1.1 404 Not Found 98ms]
TypeError: item is undefined load.php:57

Version: unspecified
Severity: normal