Filed by: aciure [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=252029955&repo=mozilla-beta
Full log: https://queue.taskcluster.net/v1/task/A8n5kFqjSomCsRSh6EqXeQ/runs/0/artifacts/public/logs/live_backing.log

11:15:47 INFO - TEST-PASS | /webdriver/tests/fullscreen_window/user_prompts.py | test_dismiss_and_notify[capabilities0-prompt-None]
11:15:47 INFO - TEST-UNEXPECTED-ERROR | /webdriver/tests/fullscreen_window/user_prompts.py | test_ignore[capabilities0-alert] - setup error
11:15:47 INFO - capabilities = {'unhandledPromptBehavior': 'ignore'}
11:15:47 INFO - configuration = {'capabilities': {'moz:firefoxOptions': {'binary': '/Users/cltbld/tasks/task_1560596402/build/application/Firefox.app/...orm.test,xn--lve-6lad.www2.not-web-platform.test,www2.www.not-web-platform.test'}}}, 'host': '127.0.0.1', 'port': 4445}
11:15:47 INFO - request = <SubRequest 'session' for <Function 'test_ignore[capabilities0-alert]'>>
11:15:47 INFO -
11:15:47 INFO - @pytest.fixture(scope="function")
11:15:47 INFO - def session(capabilities, configuration, request):
11:15:47 INFO - """Create and start a session for a test that does not itself test session creation.
11:15:47 INFO -
11:15:47 INFO - By default the session will stay open after each test, but we always try to start a
11:15:47 INFO - new one and assume that if that fails there is already a valid session. This makes it
11:15:47 INFO - possible to recover from some errors that might leave the session in a bad state, but
11:15:47 INFO - does not demand that we start a new session per test."""
11:15:47 INFO - global _current_session
11:15:47 INFO -
11:15:47 INFO - # Update configuration capabilities with custom ones from the
11:15:47 INFO - # capabilities fixture, which can be set by tests
11:15:47 INFO - caps = copy.deepcopy(configuration["capabilities"])
11:15:47 INFO - caps.update(capabilities)
11:15:47 INFO - caps = {"alwaysMatch": caps}
11:15:47 INFO -
11:15:47 INFO - # If there is a session with different capabilities active, end it now
11:15:47 INFO - if _current_session is not None and (
11:15:47 INFO - caps != _current_session.requested_capabilities):
11:15:47 INFO - _current_session.end()
11:15:47 INFO - _current_session = None
11:15:47 INFO -
11:15:47 INFO - if _current_session is None:
11:15:47 INFO - _current_session = webdriver.Session(
11:15:47 INFO - configuration["host"],
11:15:47 INFO - configuration["port"],
11:15:47 INFO - capabilities=caps)
11:15:47 INFO - try:
11:15:47 INFO - _current_session.start()
11:15:47 INFO - except webdriver.error.SessionNotCreatedException:
11:15:47 INFO - if not _current_session.session_id:
11:15:47 INFO - raise
11:15:47 INFO -
11:15:47 INFO - # Enforce a fixed default window size and position

Looks like that WebDriver:SetWindowRect caused a unexpected shutdown of Firefox due to an IPC error:

11:15:38 INFO - PID 959 | 1560597338553 webdriver::server DEBUG Deleting session
11:15:38 INFO - PID 959 | Exiting due to channel error.
11:15:38 INFO - PID 959 | Exiting due to channel error.
11:15:38 INFO - PID 959 | 1560597338660 geckodriver::marionette DEBUG Browser process stopped: exit code: 1
11:15:38 INFO - PID 959 | 1560597338667 webdriver::server DEBUG <- 500 Internal Server Error {"value":{"error":"unknown error","message":"Failed to decode response from marionette","stacktrace":""}}

Sadly due to bug 1490906 we still don't report crashes.

https://wiki.mozilla.org/Bug_Triage#Intermittent_Test_Failure_Cleanup