Filed by: ccoroiu [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=307641760&repo=mozilla-central
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/KcYusBrHTFWqQlrLnN6Ssw/runs/0/artifacts/public/logs/live_backing.log

task 2020-06-26T11:22:19.210Z] 11:22:19 INFO - TEST-PASS | browser/components/sessionstore/test/browser_491168.js | document.referrer is still correct after closing and reopening the tab. - "http://example.org/?0.891507343485298" == "http://example.org/?0.891507343485298" -
[task 2020-06-26T11:22:19.211Z] 11:22:19 INFO - Console message: [JavaScript Error: "TypeError: can't access property "legacySHistory", docShell.QueryInterface(...).sessionHistory is null" {file: "resource://gre/modules/sessionstore/SessionHistory.jsm" line: 54}]
[task 2020-06-26T11:22:19.211Z] 11:22:19 INFO - restore@resource://gre/modules/sessionstore/SessionHistory.jsm:54:16
[task 2020-06-26T11:22:19.211Z] 11:22:19 INFO - restoreHistory@resource:///modules/sessionstore/ContentRestore.jsm:164:22
[task 2020-06-26T11:22:19.211Z] 11:22:19 INFO - restoreHistory@resource:///modules/sessionstore/ContentSessionStore.jsm:639:25
[task 2020-06-26T11:22:19.211Z] 11:22:19 INFO - receiveMessage@resource:///modules/sessionstore/ContentSessionStore.jsm:570:14
[task 2020-06-26T11:22:19.211Z] 11:22:19 INFO -
[task 2020-06-26T11:22:19.211Z] 11:22:19 INFO - Console message: [JavaScript Error: "TypeError: can't access property "legacySHistory", webNavigation.sessionHistory is null" {file: "resource:///modules/sessionstore/ContentRestore.jsm" line: 242}]
[task 2020-06-26T11:22:19.211Z] 11:22:19 INFO - restoreTabContent@resource:///modules/sessionstore/ContentRestore.jsm:242:19
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - restoreTabContent@resource:///modules/sessionstore/ContentSessionStore.jsm:759:44
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - receiveMessage@resource:///modules/sessionstore/ContentSessionStore.jsm:609:16
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO -
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - Buffered messages logged at 11:21:35
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - Console message: [JavaScript Error: "TypeError: can't access property "legacySHistory", history is null" {file: "resource://gre/modules/sessionstore/SessionHistory.jsm" line: 109}]
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - collect@resource://gre/modules/sessionstore/SessionHistory.jsm:109:1
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - collect@resource://gre/modules/sessionstore/SessionHistory.jsm:39:35
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - collectFrom/<@resource:///modules/sessionstore/ContentSessionStore.jsm:200:36
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - send@resource:///modules/sessionstore/ContentSessionStore.jsm:469:19
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - sendWhenIdle@resource:///modules/sessionstore/ContentSessionStore.jsm:430:14
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO - sendWhenIdle/<@resource:///modules/sessionstore/ContentSessionStore.jsm:437:48
[task 2020-06-26T11:22:19.212Z] 11:22:19 INFO -
[task 2020-06-26T11:22:19.213Z] 11:22:19 INFO - Buffered messages logged at 11:21:54
[task 2020-06-26T11:22:19.213Z] 11:22:19 INFO - Console message: [JavaScript Error: "Unknown event: ["jsonfile", "load", "broadcastlisteners"]"]
[task 2020-06-26T11:22:19.213Z] 11:22:19 INFO - Buffered messages finished
[task 2020-06-26T11:22:19.213Z] 11:22:19 INFO - TEST-UNEXPECTED-FAIL | browser/components/sessionstore/test/browser_491168.js | Test timed out -
[task 2020-06-26T11:22:19.213Z] 11:22:19 INFO - GECKO(12024) | MEMORY STAT | vsize 2104507MB | vsizeMaxContiguous 66013704MB | residentFast 321MB | heapAllocated 85MB
[task 2020-06-26T11:22:19.213Z] 11:22:19 INFO - TEST-OK | browser/components/sessionstore/test/browser_491168.js | took 45056ms
[task 2020-06-26T11:22:19.213Z] 11:22:19 INFO - Not taking screenshot here: see the one that was previously logged
[task 2020-06-26T11:22:19.214Z] 11:22:19 INFO - TEST-UNEXPECTED-FAIL | browser/components/sessionstore/test/browser_491168.js | Found a tab after previous test timed out: http://example.org/ -
[task 2020-06-26T11:22:19.214Z] 11:22:19 INFO - GECKO(12024) | JavaScript error: resource:///modules/sessionstore/ContentSessionStore.jsm, line 466: Error: TelemetryStopwatch: key "FX_SESSION_RESTORE_CONTENT_COLLECT_DATA_MS" was already initialized
[task 2020-06-26T11:22:19.214Z] 11:22:19 INFO - GECKO(12024) | JavaScript error: resource://gre/modules/sessionstore/SessionHistory.jsm, line 109: TypeError: can't access property "legacySHistory", history is null
[task 2020-06-26T11:22:19.214Z] 11:22:19 INFO - checking window state

It is the same as bug 1647109.

Yes, I think it's likely the same as Bug 1647109. Please un-duplicate it and reopen if it keeps recurving now that that is fixed.