Beta Simulation:
https://treeherder.mozilla.org/jobs?repo=try&selectedTaskRun=QW_oP-QCShuCptK4M3QUeg.0&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&classifiedState=classified&revision=995b9253ec9cb21c1908c737f1b50878b1b01c8b&searchStr=dt
https://treeherder.mozilla.org/logviewer?job_id=326457028&repo=try&lineNumber=2348

[task 2021-01-12T13:52:05.274Z] 13:52:05 INFO - TEST-START | devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_debugger.js
[task 2021-01-12T13:52:05.314Z] 13:52:05 INFO - GECKO(1541) | DevTools Server for Browser Toolbox listening on port: 36157
[task 2021-01-12T13:52:05.320Z] 13:52:05 INFO - GECKO(1541) | Starting Browser Toolbox /builds/worker/workspace/build/application/firefox/firefox-bin -no-remote -foreground -profile /tmp/tmpm0Axz5.mozrunner/chrome_debugger_profile -chrome chrome://devtools/content/framework/browser-toolbox/window.html
[task 2021-01-12T13:52:05.542Z] 13:52:05 INFO - GECKO(1541) | > 1610459525536 Marionette TRACE Marionette enabled
[task 2021-01-12T13:52:05.544Z] 13:52:05 INFO - GECKO(1541) | > 1610459525536 Marionette TRACE Received observer notification profile-after-change
[task 2021-01-12T13:52:05.600Z] 13:52:05 INFO - GECKO(1541) | > 1610459525590 Marionette TRACE Received observer notification command-line-startup
[task 2021-01-12T13:52:05.640Z] 13:52:05 INFO - GECKO(1541) | > 1610459525636 Marionette TRACE Received observer notification toplevel-window-ready
[task 2021-01-12T13:52:08.511Z] 13:52:08 INFO - GECKO(1541) | > Waiting for debugger load
[task 2021-01-12T13:52:08.512Z] 13:52:08 INFO - GECKO(1541) | > Waiting on sources: http://mozilla.org/browser-toolbox-test-1610459525268.js
[task 2021-01-12T13:52:08.513Z] 13:52:08 INFO - GECKO(1541) | Finished waiting on sources: http://mozilla.org/browser-toolbox-test-1610459525268.js
[task 2021-01-12T13:52:08.513Z] 13:52:08 INFO - GECKO(1541) | Loaded, selecting the test script to debug
[task 2021-01-12T13:52:08.561Z] 13:52:08 INFO - TEST-INFO | started process screentopng
[task 2021-01-12T13:52:09.087Z] 13:52:09 INFO - TEST-INFO | screentopng: exit 0
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - Buffered messages logged at 13:52:05
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - Entering test bound runTest
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - TEST-PASS | devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_debugger.js | Browser toolbox started
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - -
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - TEST-PASS | devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_debugger.js | Has session state -
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - Buffered messages logged at 13:52:08
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - TEST-PASS | devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_debugger.js | Got transport -
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - TEST-PASS | devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_debugger.js | Connected -
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - Buffered messages finished
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - TEST-UNEXPECTED-FAIL | devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_debugger.js | Uncaught exception - at chrome://mochitests/content/browser/devtools/client/framework/browser-toolbox/test/helpers-browser-toolbox.js:146 - Error: ToolboxTask.spawn await rejected
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - Stack trace:
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - spawn@chrome://mochitests/content/browser/devtools/client/framework/browser-toolbox/test/helpers-browser-toolbox.js:146:13
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - asyncrunTest@chrome://mochitests/content/browser/devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_debugger.js:78:21
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - AsyncTester_execTest/<@chrome://mochikit/content/browser-test.js:1069:34
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - Tester_execTest@chrome://mochikit/content/browser-test.js:1109:11
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - nextTest/<@chrome://mochikit/content/browser-test.js:932:14
[task 2021-01-12T13:52:09.088Z] 13:52:09 INFO - SimpleTest.waitForFocus/waitForFocusInner/focusedOrLoaded/<@chrome://mochikit/content/tests/SimpleTest/SimpleTest.js:1037:23
[task 2021-01-12T13:52:09.096Z] 13:52:09 INFO - Leaving test bound runTest
[task 2021-01-12T13:52:09.096Z] 13:52:09 INFO - TEST-PASS | devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_debugger.js | The main process DevToolsServer has no pending connection when the test ends -
[task 2021-01-12T13:52:09.096Z] 13:52:09 INFO - GECKO(1541) | MEMORY STAT | vsize 2839MB | residentFast 306MB | heapAllocated 108MB
[task 2021-01-12T13:52:09.096Z] 13:52:09 INFO - TEST-OK | devtools/client/framework/browser-toolbox/test/browser_browser_toolbox_debugger.js | took 3310ms
[task 2021-01-12T13:52:09.096Z] 13:52:09 INFO - checking window state
[task 2021-01-12T13:52:09.096Z] 13:52:09 INFO - GECKO(1541) | must wait for focus

Will take a look

initToolboxTask takes an optional enableBrowserToolboxFission option which will enable the preference for the MultiprocessBrowserToolbox.
This is mandatory if the test tries to access information from the content process.

Passing the option should fix the issue.

Will make sure the MBT is enabled in channels where the corresponding preference is false (everywhere except Nightly)

https://hg.mozilla.org/mozilla-central/rev/023f2e1fe09f

Verified fixed in the latest central-as-early-beta simulation:
https://treeherder.mozilla.org/jobs?repo=try&group_state=expanded&revision=688ce23082a62ee4202b6874047fe5f152c9699f&searchStr=devtools&test_paths=devtools%2Fclient%2Fframework%2Fbrowser-toolbox%2Ftest