failing tests when run against firefox

[devoncarew]

We're seeing around 70 failures; here's an example:

test/firefox_timeouts_test.dart: TimeOuts set all timeouts (failed)
  SessionNotCreatedException (500): Session is already started
  package:webdriver/src/handler/w3c/utils.dart 81:9       parseW3cResponse
  package:webdriver/src/handler/w3c/session.dart 19:21    W3cSessionHandler.parseCreateResponse
  package:webdriver/src/common/request_client.dart 46:32  SyncRequestClient.send
  package:webdriver/sync_core.dart 61:26                  createDriver
  package:webdriver/sync_io.dart 33:10                    createDriver
  test/configs/sync_io_config.dart 40:18                  createTestDriver
  test/sync/timeouts.dart [28](https://github.com/google/webdriver.dart/actions/runs/7063764235/job/19230412646?pr=290#step:7:29):23                           runTests.<fn>.<fn>