Central as Beta: https://treeherder.mozilla.org/#/jobs?repo=try&revision=3f527b9e7d3dd625c89c9a426171ba6fef9e8821&searchStr=browser-chrome&group_state=expanded&selectedTaskRun=adKiZ8M_RCOSSSS98B7uIw.1

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=315733256&repo=try&lineNumber=10508

[task 2020-09-15T11:36:45.855Z] 11:36:45 INFO - TEST-PASS | browser/components/urlbar/tests/browser/browser_searchMode_engineRemoval.js | Found one-off button for search mode - {"engine":{"_name":"Test","_loadPath":"[other]addEngineWithDetails:Test@test.engine","description":"undefined","__searchForm":"undefined","_iconURL":"","_metaData":{"order":1},"_urls":[{"params":[],"rels":[],"resultDomain":"example.com","template":"http://example.com/?search={searchTerms}"}],"_isAppProvided":false,"_orderHint":"undefined","_telemetryId":null,"_updateInterval":null,"_updateURL":null,"_iconUpdateURL":null,"_extensionID":"Test@test.engine","_locale":"default","_definedAliases":[],"_hasPreferredIcon":null,"queryCharset":"windows-1252"},"_inheritedElements":{".button-icon":{}},"initializedAttributeInheritance":true} == true -
[task 2020-09-15T11:36:45.856Z] 11:36:45 INFO - Awaiting for the urlbar panel to open
[task 2020-09-15T11:36:45.856Z] 11:36:45 INFO - Buffered messages logged at 11:35:22
[task 2020-09-15T11:36:45.856Z] 11:36:45 INFO - Console message: [JavaScript Error: "Unknown Collection "main/nimbus-desktop-experiments"" {file: "resource://services-settings/RemoteSettingsClient.jsm" line: 160}]
[task 2020-09-15T11:36:45.856Z] 11:36:45 INFO - UnknownCollectionError@resource://services-settings/RemoteSettingsClient.jsm:160:5
[task 2020-09-15T11:36:45.856Z] 11:36:45 INFO - sync@resource://services-settings/RemoteSettingsClient.jsm:470:13
[task 2020-09-15T11:36:45.856Z] 11:36:45 INFO -
[task 2020-09-15T11:36:45.856Z] 11:36:45 INFO - Buffered messages finished
[task 2020-09-15T11:36:45.856Z] 11:36:45 INFO - TEST-UNEXPECTED-FAIL | browser/components/urlbar/tests/browser/browser_searchMode_engineRemoval.js | Test timed out -
[task 2020-09-15T11:36:45.856Z] 11:36:45 INFO - GECKO(3354) | MEMORY STAT | vsize 3684MB | residentFast 481MB | heapAllocated 141MB
[task 2020-09-15T11:36:45.864Z] 11:36:45 INFO - TEST-OK | browser/components/urlbar/tests/browser/browser_searchMode_engineRemoval.js | took 90067ms

These seem to be caused by https://hg.mozilla.org/mozilla-central/rev/af47bc7c002d4c84f857dc62b6d2046a891d2412
Harry, do you have a bit of time to check this out?
Thank you.

https://hg.mozilla.org/mozilla-central/rev/706ab0988678

Verified fixed with yesterday's central-as-beta simulation https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&group_state=expanded&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel%2Crunnable&revision=084477976b2d82a2622bc8ecda5a8e72be8b1d9e