Filed by: hskupin [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=266245574&repo=try
Full log: https://queue.taskcluster.net/v1/task/NlHIPtAIRdGxLNRFbgt-Pw/runs/0/artifacts/public/logs/live_backing.log

The actual problem here is that we are trying to access frameElement on a cross-origin object:

[task 2019-09-12T07:18:13.340Z] 07:18:13 INFO - PID 13944 | JavaScript error: chrome://marionette/content/listener.js, line 1542: SecurityError: Permission denied to access property "frameElement" on cross-origin object
[task 2019-09-12T07:18:37.896Z] 07:18:37 INFO - TEST-UNEXPECTED-TIMEOUT | /webdriver/tests/switch_to_frame/cross_origin.py | expected OK

Here the source code:

https://searchfox.org/mozilla-central/rev/a777ff11b6d700a698c61e5bd17e73b044304494/testing/marionette/listener.js

The unexpected timeout is a different problem which should not happen. As it looks like we don't catch this error anywhere, and as such hang forever in the WebDriver:SwitchToFrame command.

I will file two new bugs for Marionette this bug will depend on.

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