Filed by: apavel [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=250865367&repo=autoland
Full log: https://queue.taskcluster.net/v1/task/PB4BFRp2TLGYmqaWsbVNWA/runs/0/artifacts/public/logs/live_backing.log
Reftest URL: https://hg.mozilla.org/mozilla-central/raw-file/tip/layout/tools/reftest/reftest-analyzer.xhtml#logurl=https://queue.taskcluster.net/v1/task/PB4BFRp2TLGYmqaWsbVNWA/runs/0/artifacts/public/logs/live_backing.log&only_show_unexpected=1

[task 2019-06-10T01:30:39.726Z] 01:30:39 INFO - TEST-START | toolkit/mozapps/extensions/test/browser/browser_list.js
[task 2019-06-10T01:30:39.728Z] 01:30:39 INFO - GECKO(10461) | [Parent 10461, Main Thread] WARNING: NS_ENSURE_SUCCESS(rv, rv) failed with result 0x80520012: file /builds/worker/workspace/build/src/startupcache/StartupCacheUtils.cpp, line 203
[task 2019-06-10T01:30:39.730Z] 01:30:39 INFO - GECKO(10461) | Couldn't convert chrome URL: chrome://mochikit/content/chrome-harness.js
[task 2019-06-10T01:30:39.749Z] 01:30:39 INFO - GECKO(10461) | 1560130239746 addons.manager DEBUG Starting provider: <unnamed-provider>
[task 2019-06-10T01:30:39.756Z] 01:30:39 INFO - GECKO(10461) | 1560130239746 addons.manager DEBUG Registering shutdown blocker for <unnamed-provider>
[task 2019-06-10T01:30:39.765Z] 01:30:39 INFO - GECKO(10461) | 1560130239748 addons.manager DEBUG Provider finished startup: <unnamed-provider>
[task 2019-06-10T01:30:39.785Z] 01:30:39 INFO - GECKO(10461) | ++DOCSHELL 0x7fb20c950000 == 1 [pid = 10635] [id = {65d42673-0ace-4e02-aba4-47bccf54ad58}]

[task 2019-06-10T01:31:49.601Z] 01:31:49 INFO - TEST-INFO | Main app process: exit 0
[task 2019-06-10T01:31:49.602Z] 01:31:49 ERROR - TEST-UNEXPECTED-FAIL | toolkit/mozapps/extensions/test/browser/browser_list.js | leaked 2 window(s) until shutdown [url = about:addons]
[task 2019-06-10T01:31:49.603Z] 01:31:49 INFO - TEST-INFO | toolkit/mozapps/extensions/test/browser/browser_list.js | windows(s) leaked: [pid = 10831] [serial = 30], [pid = 10831] [serial = 33]
[task 2019-06-10T01:31:49.604Z] 01:31:49 INFO - TEST-INFO | toolkit/mozapps/extensions/test/browser/browser_list.js | This test created 1 hidden window(s)
[task 2019-06-10T01:31:49.604Z] 01:31:49 INFO - TEST-INFO | toolkit/mozapps/extensions/test/browser/browser_list.js | This test created 1 hidden docshell(s)

This is not a regression. That patch did not change the observable behavior of the test. The only change was setting the value of a pref that at the time was already at that value.

This bug will soon be obsolete anyway, when we remove the file - bug 1558982

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