Bug 1542415 got merged and started to fail on m-c:

https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&selectedJob=265736975&resultStatus=testfailed%2Cbusted%2Cexception&searchStr=m-fis%2C%285%29

Bug 1542415 seems to be the culprit:

https://treeherder.mozilla.org/#/jobs?repo=autoland&searchStr=m-fis%285%29&tochange=b5ef2b8f7c2b739502c441a776e3266b53d10f5d&fromchange=edaf6d0b4d0432d61f8cfdbacbb80d532b8374a3&selectedJob=265761872

Log link: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=265736975&repo=mozilla-central&lineNumber=5170

Log snippet:

[task 2019-09-09T16:32:23.717Z] 16:32:23 INFO - TEST-FAIL | toolkit/components/extensions/test/mochitest/test_ext_cookies.html | deleted the private cookie - Expected: null, Actual: null
[task 2019-09-09T16:32:23.717Z] 16:32:23 INFO - TEST-FAIL | toolkit/components/extensions/test/mochitest/test_ext_cookies.html | executeScript returns one result - Expected: 1, Actual: 1
[task 2019-09-09T16:32:23.718Z] 16:32:23 INFO - TEST-FAIL | toolkit/components/extensions/test/mochitest/test_ext_cookies.html | private document.cookie should be removed - Expected: , Actual:
[task 2019-09-09T16:32:23.718Z] 16:32:23 INFO - TEST-FAIL | toolkit/components/extensions/test/mochitest/test_ext_cookies.html | cookies
[task 2019-09-09T16:32:23.718Z] 16:32:23 INFO - TEST-FAIL | toolkit/components/extensions/test/mochitest/test_ext_cookies.html | test result correct
[task 2019-09-09T16:32:23.719Z] 16:32:23 INFO - add_task | Leaving test test_cookies
[task 2019-09-09T16:32:23.719Z] 16:32:23 INFO - Buffered messages finished
[task 2019-09-09T16:32:23.719Z] 16:32:23 INFO - TEST-UNEXPECTED-PASS | toolkit/components/extensions/test/mochitest/test_ext_cookies.html | fail-if condition in manifest - We expected at least one failure
[task 2019-09-09T16:32:23.720Z] 16:32:23 INFO - TEST-INFO | expected FAIL
[task 2019-09-09T16:32:23.720Z] 16:32:23 INFO - GECKO(5252) | MEMORY STAT | vsize 2498MB | residentFast 136MB | heapAllocated 15MB
[task 2019-09-09T16:32:23.720Z] 16:32:23 INFO - TEST-OK | toolkit/components/extensions/test/mochitest/test_ext_cookies.html | took 1630ms

Fixed by: https://hg.mozilla.org/integration/autoland/rev/dd9684586ba3#l8.12

Retroactively moving fixed bugs whose summaries mention "Fission" (or other Fission-related keywords) but are not assigned to a Fission Milestone to an appropriate Fission Milestone.

This will generate a lot of bugmail, so you can filter your bugmail for the following UUID and delete them en masse:

0ee3c76a-bc79-4eb2-8d12-05dc0b68e732