Filed by: rmaries [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=325507032&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/cXj2YaDJTrW8agsKdU1nNQ/runs/0/artifacts/public/logs/live_backing.log

[task 2020-12-28T17:21:07.829Z] 17:21:07     INFO - TEST-START | browser/components/sessionstore/test/browser_scrollPositions.js
[task 2020-12-28T17:21:09.207Z] 17:21:09     INFO - TEST-INFO | started process screentopng
[task 2020-12-28T17:21:09.647Z] 17:21:09     INFO - TEST-INFO | screentopng: exit 0
[task 2020-12-28T17:21:09.647Z] 17:21:09     INFO - Buffered messages logged at 17:21:07
[task 2020-12-28T17:21:09.648Z] 17:21:09     INFO - Entering test bound test_scroll
[task 2020-12-28T17:21:09.648Z] 17:21:09     INFO - Buffered messages finished
[task 2020-12-28T17:21:09.648Z] 17:21:09     INFO - TEST-UNEXPECTED-FAIL | browser/components/sessionstore/test/browser_scrollPositions.js | scroll is fine - Got "{\"scroll\":\"322,428\"}", expected "{\"scroll\":\"161,214\"}"
[task 2020-12-28T17:21:09.648Z] 17:21:09     INFO - Stack trace:
[task 2020-12-28T17:21:09.648Z] 17:21:09     INFO - chrome://mochikit/content/browser-test.js:test_is:1332
[task 2020-12-28T17:21:09.648Z] 17:21:09     INFO - chrome://mochitests/content/browser/browser/components/sessionstore/test/head.js:checkScroll:623
[task 2020-12-28T17:21:09.990Z] 17:21:09     INFO - Not taking screenshot here: see the one that was previously logged```

The failure align with the first landing + backout of bug 1676966.