central-as-beta simulation
Failure log: https://treeherder.mozilla.org/logviewer.html#?job_id=319545830&repo=try

[task 2020-10-23T12:52:12.469Z] 12:52:12 INFO - Log some messages before calling ResourceWatcher.watchResources in order to assert the behavior of already existing messages.
[task 2020-10-23T12:52:12.470Z] 12:52:12 INFO - Wait for existing browser mochitest log
[task 2020-10-23T12:52:12.471Z] 12:52:12 INFO - TEST-PASS | devtools/shared/resources/tests/browser_browser_resources_console_messages.js | The existing log was retrieved -
[task 2020-10-23T12:52:12.472Z] 12:52:12 INFO - Wait for runtime browser mochitest log
[task 2020-10-23T12:52:12.472Z] 12:52:12 INFO - TEST-PASS | devtools/shared/resources/tests/browser_browser_resources_console_messages.js | The runtime log was retrieved -
[task 2020-10-23T12:52:12.476Z] 12:52:12 INFO - Adding a new tab with URL: https://example.com/browser/devtools/shared/resources/tests/early_console_document.html
[task 2020-10-23T12:52:12.476Z] 12:52:12 INFO - Tab added and finished loading
[task 2020-10-23T12:52:12.476Z] 12:52:12 INFO - Wait for early page log
[task 2020-10-23T12:52:12.476Z] 12:52:12 INFO - Console message: [JavaScript Error: "Error: Can't find profile directory." {file: "resource://gre/modules/XULStore.jsm" line: 66}]
[task 2020-10-23T12:52:12.477Z] 12:52:12 INFO - load@resource://gre/modules/XULStore.jsm:66:15
[task 2020-10-23T12:52:12.477Z] 12:52:12 INFO - XULStore@resource://gre/modules/XULStore.jsm:24:10

(several more of these JavaScript errors)

[task 2020-10-23T12:52:12.484Z] 12:52:12 INFO - Buffered messages finished
[task 2020-10-23T12:52:12.484Z] 12:52:12 INFO - TEST-UNEXPECTED-FAIL | devtools/shared/resources/tests/browser_browser_resources_console_messages.js | Test timed out -

Looking into this (I'm reproducing the failure on beta sim locally)

This is flipping the devtools.browsertoolbox.fission pref, which
is only enabled on Nightly at the moment.

The severity field is not set for this bug.
:ochameau, could you have a look please?

For more information, please visit auto_nag documentation.

Please land the patch if it is ready to land in its current state.

https://hg.mozilla.org/mozilla-central/rev/e2c9ef0e1235

Verified fixed on today's beta sim.

https://treeherder.mozilla.org/jobs?repo=try&revision=dd562f0d3ea27e27e02880bd123e8e04811a6566&searchStr=Linux%2C18.04%2Cx64%2Cshippable%2Copt%2CMochitests%2Ctest-linux1804-64-shippable%2Fopt-mochitest-devtools-chrome-e10s%2Cdt3&selectedTaskRun=cveAlsoiSmStJEnrJshr4w.0

(In reply to Sebastian Hengst [:aryx] (needinfo on intermittent or backout) from comment #5)

Please land the patch if it is ready to land in its current state.

Done :)