Filed by: rmaries [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=317368719&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/eJ-PnFFkR9CG03fPwkGuuA/runs/0/artifacts/public/logs/live_backing.log

[task 2020-10-02T06:53:59.015Z] 06:53:59     INFO - TEST-START | /intersection-observer/same-origin-grand-child-iframe.sub.html
[task 2020-10-02T06:53:59.025Z] 06:53:59     INFO - Closing window 8589934663
[task 2020-10-02T06:53:59.444Z] 06:53:59     INFO - 
[task 2020-10-02T06:53:59.444Z] 06:53:59     INFO - TEST-UNEXPECTED-FAIL | /intersection-observer/same-origin-grand-child-iframe.sub.html | rootBounds in a same-origin iframe in the case where there is a cross-origin iframe in between the top document and the same origin iframe - assert_equals: expected 292 but got 300
[task 2020-10-02T06:53:59.444Z] 06:53:59     INFO - @http://web-platform.test:8000/intersection-observer/same-origin-grand-child-iframe.sub.html:23:16
[task 2020-10-02T06:53:59.444Z] 06:53:59     INFO - TEST-OK | /intersection-observer/same-origin-grand-child-iframe.sub.html | took 426ms
[task 2020-10-02T06:53:59.449Z] 06:53:59     INFO - PID 9928 | 1601621639446	Marionette	INFO	Stopped listening on port 52956
[task 2020-10-02T06:53:59.816Z] 06:53:59     INFO - Browser exited with return code 0
[task 2020-10-02T06:53:59.817Z] 06:53:59     INFO - PROCESS LEAKS None
[task 2020-10-02T06:53:59.819Z] 06:53:59     INFO - Closing logging queue
[task 2020-10-02T06:53:59.819Z] 06:53:59     INFO - queue closed
[task 2020-10-02T06:53:59.887Z] 06:53:59     INFO - Application command: C:\Users\task_1601620822\build\application\firefox\firefox.exe -marionette about:blank --wait-for-browser -profile c:\users\task_1601620822\appdata\local\temp\tmpva04d3
[task 2020-10-02T06:53:59.889Z] 06:53:59     INFO - PID 5596 | Can't find symbol 'eglSwapBuffersWithDamageEXT'.
[task 2020-10-02T06:53:59.889Z] 06:53:59     INFO - PID 5596 | console.warn: SearchSettings: "get: No settings file exists, new profile?" (new Error("", "(unknown module)"))
[task 2020-10-02T06:53:59.889Z] 06:53:59     INFO - PID 5596 | 1601621631598	Marionette	INFO	Listening on port 53001```

I think this should've been fixed by bug 1668784 (this is the case that crashed in a debug build). Please reopen and ni? if not :)