Filed by: abutkovits [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=342243715&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/cVN6qCA1Q1GyyZ8pRHCKtA/runs/0/artifacts/public/logs/live_backing.log

[task 2021-06-09T11:07:55.781Z] 11:07:55     INFO - TEST-START | devtools/client/application/test/browser/browser_application_panel_list-several-workers.js
[task 2021-06-09T11:07:56.294Z] 11:07:56     INFO - GECKO(1549) | Empty service worker registered
[task 2021-06-09T11:07:56.849Z] 11:07:56     INFO - GECKO(1549) | console.warn: "Listener for event 'frame' did not return a promise."
[task 2021-06-09T11:08:40.792Z] 11:08:40     INFO - TEST-INFO | started process screentopng
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - TEST-INFO | screentopng: exit 0
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Buffered messages logged at 11:07:55
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Entering test bound 
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Adding a new tab with URL: http://example.com/browser/devtools/client/application/test/browser/resources/service-workers/simple.html
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Buffered messages logged at 11:07:56
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Tab added and finished loading
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Selecting application page: service-workers
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Wait until the service worker appears in the application panel
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Wait until the unregister button is displayed for the service worker
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - TEST-PASS | devtools/client/application/test/browser/browser_application_panel_list-several-workers.js | First service worker registration is displayed - 
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Navigate to another page for the same domain with another service worker
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Navigating to "http://example.com/browser/devtools/client/application/test/browser/resources/service-workers/scope-page.html"
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Waiting for page to be loaded…
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - → page loaded
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Waiting for target switch…
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - → switched-target emitted
[task 2021-06-09T11:08:41.112Z] 11:08:41     INFO - Wait until the service worker appears in the application panel
[task 2021-06-09T11:08:41.113Z] 11:08:41     INFO - Buffered messages finished
[task 2021-06-09T11:08:41.114Z] 11:08:41     INFO - TEST-UNEXPECTED-FAIL | devtools/client/application/test/browser/browser_application_panel_list-several-workers.js | Test timed out -

Fixed by backout of bug 1715300.