Filed by: nerli [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=281669780&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/eRiVTJ20T7-LD3_1IVOebA/runs/0/artifacts/public/logs/live_backing.log

[task 2019-12-18T03:12:43.846Z] 03:12:43 INFO - STDOUT: Module already imported so cannot be rewritten: mozlog
[task 2019-12-18T03:12:43.846Z] 03:12:43 INFO - STDOUT: -- Docs: http://doc.pytest.org/en/latest/warnings.html
[task 2019-12-18T03:12:43.847Z] 03:12:43 INFO - STDOUT: ================ 1 passed, 1 warnings, 1 error in 8.52 seconds ================
[task 2019-12-18T03:12:43.854Z] 03:12:43 INFO -
[task 2019-12-18T03:12:43.854Z] 03:12:43 INFO - TEST-UNEXPECTED-ERROR | /webdriver/tests/delete_all_cookies/delete.py | test_null_response_value - setup error
[task 2019-12-18T03:12:43.854Z] 03:12:43 INFO - capabilities = {}
[task 2019-12-18T03:12:43.854Z] 03:12:43 INFO - configuration = {'capabilities': {'moz:firefoxOptions': {'binary': 'Z:\task_1576637543\build\application\firefox\firefox.exe', 'prefs'...platform.test,www2.www.not-web-platform.test', 'network.process.enabled': False}}}, 'host': '127.0.0.1', 'port': 49909}
[task 2019-12-18T03:12:43.854Z] 03:12:43 INFO - request = <SubRequest 'session' for <Function 'test_null_response_value'>>
[task 2019-12-18T03:12:43.854Z] 03:12:43 INFO -
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - @pytest.fixture(scope="function")
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - def session(capabilities, configuration, request):
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - """Create and start a session for a test that does not itself test session creation.
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO -
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - By default the session will stay open after each test, but we always try to start a
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - new one and assume that if that fails there is already a valid session. This makes it
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - possible to recover from some errors that might leave the session in a bad state, but
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - does not demand that we start a new session per test."""
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - global _current_session
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO -
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - # Update configuration capabilities with custom ones from the
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - # capabilities fixture, which can be set by tests
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - caps = copy.deepcopy(configuration["capabilities"])
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - caps.update(capabilities)
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO - caps = {"alwaysMatch": caps}
[task 2019-12-18T03:12:43.855Z] 03:12:43 INFO -
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - # If there is a session with different capabilities active, end it now
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - if _current_session is not None and (
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - caps != _current_session.requested_capabilities):
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - _current_session.end()
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - _current_session = None
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO -
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - if _current_session is None:
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - _current_session = webdriver.Session(
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - configuration["host"],
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - configuration["port"],
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - capabilities=caps)
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - try:
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - > _current_session.start()
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO -
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - capabilities = {}
[task 2019-12-18T03:12:43.856Z] 03:12:43 INFO - caps = {'alwaysMatch': {'moz:firefoxOptions': {'binary': 'Z:\task_1576637543\build\application\firefox\firefox.exe', 'prefs':...form.test,xn--lve-6lad.www2.not-web-platform.test,www2.www.not-web-platform.test', 'network.process.enabled': False}}}}
[task 2019-12-18T03:12:43.857Z] 03:12:43 INFO - configuration = {'capabilities': {'moz:firefoxOptions': {'binary': 'Z:\task_1576637543\build\application\firefox\firefox.exe', 'prefs'...platform.test,www2.www.not-web-platform.test', 'network.process.enabled': False}}}, 'host': '127.0.0.1', 'port': 49909}
[task 2019-12-18T03:12:43.857Z] 03:12:43 INFO - request = <SubRequest 'session' for <Function 'test_null_response_value'>>
[task 2019-12-18T03:12:43.857Z] 03:12:43 INFO -
[task 2019-12-18T03:12:43.857Z] 03:12:43 INFO - tests\web-platform\tests\webdriver\tests\support\fixtures.py:150:
[task 2019-12-18T03:12:43.857Z] 03:12:43 INFO - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
[task 2019-12-18T03:12:43.857Z] 03:12:43 INFO - tests\web-platform\tests\tools\webdriver\webdriver\client.py:428: in start
[task 2019-12-18T03:12:43.857Z] 03:12:43 INFO - value = self.send_command("POST", "session", body=body)
[task 2019-12-18T03:12:43.857Z] 03:12:43 INFO - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

geckodriver kills Firefox again after 1s of waiting:

[task 2019-12-18T03:12:35.357Z] 03:12:35     INFO - PID 3508 | 1576638755348	geckodriver::marionette	DEBUG	Waiting 60s to connect to browser on 127.0.0.1:2828
[task 2019-12-18T03:12:36.457Z] 03:12:36     INFO - PID 3508 | 1576638756449	mozrunner::runner	DEBUG	Killing process 5364
[task 2019-12-18T03:12:36.457Z] 03:12:36     INFO - PID 3508 | 1576638756449	webdriver::server	DEBUG	<- 500 Internal Server Error {"value":{"error":"unknown error","message":"permission denied","stacktrace":""}}

https://wiki.mozilla.org/Bug_Triage#Intermittent_Test_Failure_Cleanup
For more information, please visit auto_nag documentation.