Filed by: ncsoregi [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=338852104&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/fEJkPtvJSqqBJWwVVDueow/runs/0/artifacts/public/logs/live_backing.log

[task 2021-05-05T19:01:34.102Z] 19:01:33     INFO - TEST-PASS | browser/components/sessionstore/test/browser_reopen_all_windows.js | ["about:blank","http://sub1.test1.mochi.test:8888/browser/browser/components/sessionstore/test/empty.html","http://sub2.xn--lt-uia.mochi.test:8888/browser/browser/components/sessionstore/test/empty.html","http://test2.mochi.test:8888/browser/browser/components/sessionstore/test/empty.html","http://sub1.test2.example.org/browser/browser/components/sessionstore/test/empty.html","http://sub2.test1.example.org/browser/browser/components/sessionstore/test/empty.html","http://test2.example.com/browser/browser/components/sessionstore/test/empty.html"] deepEqual ["about:blank","http://sub1.test1.mochi.test:8888/browser/browser/components/sessionstore/test/empty.html","http://sub2.xn--lt-uia.mochi.test:8888/browser/browser/components/sessionstore/test/empty.html","http://test2.mochi.test:8888/browser/browser/components/sessionstore/test/empty.html","http://sub1.test2.example.org/browser/browser/components/sessionstore/test/empty.html","http://sub2.test1.example.org/browser/browser/components/sessionstore/test/empty.html","http://test2.example.com/browser/browser/components/sessionstore/test/empty.html"] - 
[task 2021-05-05T19:01:49.992Z] 19:01:48     INFO - About to close windows
[task 2021-05-05T19:01:56.447Z] 19:01:55     INFO - Buffered messages finished
[task 2021-05-05T19:01:58.513Z] 19:01:56    ERROR - TEST-UNEXPECTED-TIMEOUT | browser/components/sessionstore/test/browser_reopen_all_windows.js | application timed out after 370 seconds with no output
[task 2021-05-05T19:02:00.591Z] 19:01:58    ERROR - Force-terminating active process(es).
[task 2021-05-05T19:02:07.704Z] 19:02:07     INFO - Determining child pids from psutil...
[task 2021-05-05T19:04:09.238Z] 19:04:09     INFO - [1647, 1671, 1726, 1765, 1803, 1833, 1922, 1955, 2074]
[task 2021-05-05T19:04:09.288Z] 19:04:09     INFO - ==> process 1569 launched child process 1584
[task 2021-05-05T19:04:09.301Z] 19:04:09     INFO - ==> process 1569 launched child process 1647
[task 2021-05-05T19:04:09.302Z] 19:04:09     INFO - ==> process 1569 launched child process 1671
[task 2021-05-05T19:04:09.303Z] 19:04:09     INFO - ==> process 1569 launched child process 1726
[task 2021-05-05T19:04:09.303Z] 19:04:09     INFO - ==> process 1569 launched child process 1765
[task 2021-05-05T19:04:09.303Z] 19:04:09     INFO - ==> process 1569 launched child process 1803
[task 2021-05-05T19:04:09.303Z] 19:04:09     INFO - ==> process 1569 launched child process 1833
[task 2021-05-05T19:04:09.303Z] 19:04:09     INFO - ==> process 1569 launched child process 1922
[task 2021-05-05T19:04:09.303Z] 19:04:09     INFO - ==> process 1569 launched child process 2074
[task 2021-05-05T19:04:09.303Z] 19:04:09     INFO - Found child pids: {1922, 1955, 1765, 1671, 1833, 1803, 1647, 1584, 2074, 1726}
[task 2021-05-05T19:04:09.314Z] 19:04:09     INFO - Failed to get child procs
[task 2021-05-05T19:04:09.315Z] 19:04:09     INFO - Killing process: 1922
[task 2021-05-05T19:04:09.338Z] 19:04:09     INFO - TEST-INFO | started process screentopng
[task 2021-05-05T19:04:10.365Z] 19:04:10     INFO - TEST-INFO | screentopng: exit 0```

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