Filed by: nbeleuzu [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=360858653&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/XdX6IgTjR6qvS76SIJaf8A/runs/0/artifacts/public/logs/live_backing.log

[task 2021-12-11T00:36:29.362Z] 00:36:29     INFO - TEST-PASS | dom/security/test/csp/test_block_all_mixed_content.html | (no-csp) mixed display content should be loaded 
[task 2021-12-11T00:36:29.363Z] 00:36:29     INFO - Buffered messages finished
[task 2021-12-11T00:36:29.364Z] 00:36:29     INFO - TEST-UNEXPECTED-FAIL | dom/security/test/csp/test_block_all_mixed_content.html | Test timed out. - 
[task 2021-12-11T00:36:30.219Z] 00:36:30     INFO - GECKO(12905) | MEMORY STAT | vsize 2497MB | residentFast 109MB | heapAllocated 6MB
[task 2021-12-11T00:36:30.225Z] 00:36:30     INFO - TEST-OK | dom/security/test/csp/test_block_all_mixed_content.html | took 326123ms

Any idea what started this Linux opt fission failure on Friday evening?

I went through our team's recent patches, but there are only two in question and they are both not related (bug 1742899 and bug 1744924)
Maybe there was a pref flip or some other default change in another component that affects image loading or such things?
The test in itself doesn't look racy at all.

I went through https://hg.mozilla.org/integration/autoland/pushloghtml?startdate=2021-12-09&enddate=2021-12-11, but I was also wondering if there's a more granular regression range we could get?

This looks like a variant of bug 1701828 without the second failure line.

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