Filed by: dvarga [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=287619400&repo=mozilla-central
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/PPSX2qXYTfCATEFxMyHByQ/runs/0/artifacts/public/logs/live_backing.log

[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - TEST-PASS | widget/tests/browser/browser_test_clipboardcache.js | Private read - 
[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - Buffered messages finished
[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - TEST-UNEXPECTED-FAIL | widget/tests/browser/browser_test_clipboardcache.js | Test timed out - 
[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - GECKO(16889) | MEMORY STAT vsizeMaxContiguous not supported in this build configuration.
[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - GECKO(16889) | MEMORY STAT | vsize 3293MB | residentFast 592MB | heapAllocated 212MB
[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - TEST-OK | widget/tests/browser/browser_test_clipboardcache.js | took 45071ms
[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - checking window state
[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - Not taking screenshot here: see the one that was previously logged
[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - TEST-UNEXPECTED-FAIL | widget/tests/browser/browser_test_clipboardcache.js | Found a browser window after previous test timed out - 
[task 2020-02-05T18:24:57.507Z] 18:24:57     INFO - GECKO(16889) | must wait for focus
[task 2020-02-05T18:24:58.410Z] 18:24:58     INFO - GECKO(16889) | [CodeCoverage] Setting handlers for process 17169.
[task 2020-02-05T18:25:03.726Z] 18:25:03     INFO - Not taking screenshot here: see the one that was previously logged
[task 2020-02-05T18:25:03.728Z] 18:25:03     INFO - TEST-UNEXPECTED-FAIL | widget/tests/browser/browser_test_clipboardcache.js | Uncaught exception received from previously timed out test - [Exception... "(null)"  nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)"  location: "<unknown>"  data: no]
[task 2020-02-05T18:25:04.940Z] 18:25:04     INFO - GECKO(16889) | Completed ShutdownLeaks collections in process 17142
[task 2020-02-05T18:25:05.001Z] 18:25:05     INFO - GECKO(16889) | Completed ShutdownLeaks collections in process 17088
[task 2020-02-05T18:25:05.089Z] 18:25:05     INFO - GECKO(16889) | Completed ShutdownLeaks collections in process 16975
[task 2020-02-05T18:25:05.092Z] 18:25:05     INFO - GECKO(16889) | Completed ShutdownLeaks collections in process 17032
[task 2020-02-05T18:25:05.133Z] 18:25:05     INFO - GECKO(16889) | Completed ShutdownLeaks collections in process 17062
[task 2020-02-05T18:25:05.214Z] 18:25:05     INFO - GECKO(16889) | Completed ShutdownLeaks collections in process 16950
[task 2020-02-05T18:25:07.409Z] 18:25:07     INFO - GECKO(16889) | Completed ShutdownLeaks collections in process 16889

Could you clarify what information is needed from me?

Depends on D61994

https://hg.mozilla.org/mozilla-central/rev/692954bc2a1e