Tests failing with Firefox Nightly

[marco-c]

  10) Selenium Firefox Nightly
       send/receive notification without payload with Firefox Nightly (aes128gcm):
     Error retrieving push subscription.
  Async*@http://127.0.0.1:8000/:52:28
  promise callback*@http://127.0.0.1:8000/:34:4
  @http://127.0.0.1:8000/:63:3

subscribeError:  {
  ABORT_ERR: 20,
  DATA_CLONE_ERR: 25,
  DOMSTRING_SIZE_ERR: 2,
  HIERARCHY_REQUEST_ERR: 3,
  INDEX_SIZE_ERR: 1,
  INUSE_ATTRIBUTE_ERR: 10,
  INVALID_ACCESS_ERR: 15,
  INVALID_CHARACTER_ERR: 5,
  INVALID_MODIFICATION_ERR: 13,
  INVALID_NODE_TYPE_ERR: 24,
  INVALID_STATE_ERR: 11,
  NAMESPACE_ERR: 14,
  NETWORK_ERR: 19,
  NOT_FOUND_ERR: 8,
  NOT_SUPPORTED_ERR: 9,
  NO_DATA_ALLOWED_ERR: 6,
  NO_MODIFICATION_ALLOWED_ERR: 7,
  QUOTA_EXCEEDED_ERR: 22,
  SECURITY_ERR: 18,
  SYNTAX_ERR: 12,
  TIMEOUT_ERR: 23,
  TYPE_MISMATCH_ERR: 17,
  URL_MISMATCH_ERR: 21,
  VALIDATION_ERR: 16,
  WRONG_DOCUMENT_ERR: 4,
  code: 20,
  columnNumber: 0,
  data: null,
  filename: 'http://127.0.0.1:8000/',
  lineNumber: 52,
  message: 'Error retrieving push subscription.',
  name: 'AbortError',
  result: 2152923156,
  stack: 'Async*@http://127.0.0.1:8000/:52:28\npromise ' +
    'callback*@http://127.0.0.1:8000/:34:4\n' +
    '@http://127.0.0.1:8000/:63:3\n'
}