Filed by: malexandru [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=332999086&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/dfG3saErQF68L_pOXBmEtQ/runs/0/artifacts/public/logs/live_backing.log

[task 2021-03-12T12:36:04.267Z] 12:36:04     INFO - TEST-OK | browser/components/sessionstore/test/browser_windowRestore_perwindowpb.js | took 814ms
[task 2021-03-12T12:36:04.272Z] 12:36:04     INFO - checking window state
[task 2021-03-12T12:36:04.272Z] 12:36:04     INFO - TEST-START | browser/components/sessionstore/test/browser_windowStateContainer.js
[task 2021-03-12T12:37:34.503Z] 12:37:34     INFO - TEST-INFO | started process screencapture
[task 2021-03-12T12:37:34.597Z] 12:37:34     INFO - TEST-INFO | screencapture: exit 0
[task 2021-03-12T12:37:34.597Z] 12:37:34     INFO - Buffered messages logged at 12:36:04
[task 2021-03-12T12:37:34.597Z] 12:37:34     INFO - Entering test bound setup
[task 2021-03-12T12:37:34.597Z] 12:37:34     INFO - Leaving test bound setup
[task 2021-03-12T12:37:34.597Z] 12:37:34     INFO - Entering test bound 
[task 2021-03-12T12:37:34.597Z] 12:37:34     INFO - Buffered messages logged at 12:36:06
[task 2021-03-12T12:37:34.600Z] 12:37:34     INFO - TEST-PASS | browser/components/sessionstore/test/browser_windowStateContainer.js | 1st Window: tabs[0].userContextId should exist. - 1 == 1 - 
[task 2021-03-12T12:37:34.600Z] 12:37:34     INFO - TEST-PASS | browser/components/sessionstore/test/browser_windowStateContainer.js | 1st Window: tabs[1].userContextId should exist. - 2 == 2 - 
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - TEST-PASS | browser/components/sessionstore/test/browser_windowStateContainer.js | 1st Window: tabs[2].userContextId should exist. - 3 == 3 - 
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - TEST-PASS | browser/components/sessionstore/test/browser_windowStateContainer.js | 1st Window: tabs[3].userContextId should exist. - 4 == 4 - 
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - Buffered messages logged at 12:36:07
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - TEST-PASS | browser/components/sessionstore/test/browser_windowStateContainer.js | The docShell has the correct userContextId - 1 == 1 - 
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - TEST-PASS | browser/components/sessionstore/test/browser_windowStateContainer.js | The document has the correct userContextId - 1 == 1 - 
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - TEST-PASS | browser/components/sessionstore/test/browser_windowStateContainer.js | The docShell has the correct userContextId - 2 == 2 - 
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - TEST-PASS | browser/components/sessionstore/test/browser_windowStateContainer.js | The document has the correct userContextId - 2 == 2 - 
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - Buffered messages logged at 12:36:49
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - Longer timeout required, waiting longer...  Remaining timeouts: 1
[task 2021-03-12T12:37:34.601Z] 12:37:34     INFO - Buffered messages finished
[task 2021-03-12T12:37:34.604Z] 12:37:34     INFO - TEST-UNEXPECTED-FAIL | browser/components/sessionstore/test/browser_windowStateContainer.js | Test timed out - 
[task 2021-03-12T12:37:34.604Z] 12:37:34     INFO - GECKO(2862) | MEMORY STAT | vsize 8686MB | residentFast 708MB | heapAllocated 145MB
[task 2021-03-12T12:37:34.604Z] 12:37:34     INFO - TEST-OK | browser/components/sessionstore/test/browser_windowStateContainer.js | took 90242ms
[task 2021-03-12T12:37:34.604Z] 12:37:34     INFO - checking window state
[task 2021-03-12T12:37:34.604Z] 12:37:34     INFO - Not taking screenshot here: see the one that was previously logged
[task 2021-03-12T12:37:34.604Z] 12:37:34     INFO - TEST-UNEXPECTED-FAIL | browser/components/sessionstore/test/browser_windowStateContainer.js | Found a browser window after previous test timed out - 
[task 2021-03-12T12:37:34.613Z] 12:37:34     INFO - Not taking screenshot here: see the one that was previously logged
[task 2021-03-12T12:37:34.613Z] 12:37:34     INFO - TEST-UNEXPECTED-FAIL | browser/components/sessionstore/test/browser_windowStateContainer.js | Found a browser window after previous test timed out - 
[task 2021-03-12T12:37:34.613Z] 12:37:34     INFO - GECKO(2862) | must wait for focus
[task 2021-03-12T12:37:34.723Z] 12:37:34     INFO - GECKO(2862) | Completed ShutdownLeaks collections in process 2862
[task 2021-03-12T12:37:34.723Z] 12:37:34     INFO - TEST-START | Shutdown```

Should be fixed by bug 1703556.

Bug 1703556 should have fixed this.