Filed by: abutkovits [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=342259814&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/IMiJd30WQAKDnSBuvx8Jsw/runs/0/artifacts/public/logs/live_backing.log

[task 2021-06-09T13:31:51.241Z] 13:31:51     INFO - TEST-PASS | browser/components/extensions/test/browser/browser_ext_sessions_getRecentlyClosed.js | each item has a unique sessionId - 
[task 2021-06-09T13:31:51.241Z] 13:31:51     INFO - Buffered messages logged at 13:31:48
[task 2021-06-09T13:31:51.242Z] 13:31:51     INFO - TEST-PASS | browser/components/extensions/test/browser/browser_ext_sessions_getRecentlyClosed.js | most recently closed window has the expected number of tabs - 
[task 2021-06-09T13:31:51.242Z] 13:31:51     INFO - Buffered messages finished
[task 2021-06-09T13:31:51.244Z] 13:31:51     INFO - TEST-UNEXPECTED-FAIL | browser/components/extensions/test/browser/browser_ext_sessions_getRecentlyClosed.js | the expected number of closed tabs/windows was found - Got 4, expected 5
[task 2021-06-09T13:31:51.244Z] 13:31:51     INFO - Stack trace:
[task 2021-06-09T13:31:51.244Z] 13:31:51     INFO - chrome://mochikit/content/browser-test.js:test_is:1362
[task 2021-06-09T13:31:51.244Z] 13:31:51     INFO - chrome://mochitests/content/browser/browser/components/extensions/test/browser/head_sessions.js:checkRecentlyClosed:49
[task 2021-06-09T13:31:51.244Z] 13:31:51     INFO - chrome://mochitests/content/browser/browser/components/extensions/test/browser/browser_ext_sessions_getRecentlyClosed.js:test_sessions_get_recently_closed:94
[task 2021-06-09T13:31:51.244Z] 13:31:51     INFO - TEST-PASS | browser/components/extensions/test/browser/browser_ext_sessions_getRecentlyClosed.js | closed tab has the expected value for highlighted - 

Fixed by backout of bug 1715300.