I've been investigating this in an attempt to land the patches for bug 1567230. It appears that the master password prompt is closing before event listener for DOMModalDialogClosed event is added, though moving it sooner within the .then() doesn't fix it.

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception&fromchange=755c7d187626dd1be04708d94c19ddfc99662823&tochange=af37a24bac5db7671dd5394173584789567d2304&selectedJob=258428041

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=258428041&repo=autoland&lineNumber=2981

[task 2019-07-26T01:46:37.013Z] 01:46:37 INFO - TEST-PASS | browser/components/aboutlogins/tests/browser/browser_masterPassword.js | Logins should be displayed when MP is set and authenticated -
[task 2019-07-26T01:46:37.018Z] 01:46:37 INFO - TEST-PASS | browser/components/aboutlogins/tests/browser/browser_masterPassword.js | Dialog is the Master Password dialog -
[task 2019-07-26T01:46:37.019Z] 01:46:37 INFO - Master Password dialog shown and canceled
[task 2019-07-26T01:46:37.021Z] 01:46:37 INFO - Clicking copy password button again
[task 2019-07-26T01:46:37.022Z] 01:46:37 INFO - TEST-PASS | browser/components/aboutlogins/tests/browser/browser_masterPassword.js | Dialog is the Master Password dialog -
[task 2019-07-26T01:46:37.023Z] 01:46:37 INFO - Buffered messages finished
[task 2019-07-26T01:46:37.025Z] 01:46:37 INFO - TEST-UNEXPECTED-FAIL | browser/components/aboutlogins/tests/browser/browser_masterPassword.js | Test timed out -
[task 2019-07-26T01:46:37.026Z] 01:46:37 INFO - GECKO(2216) | MP change from omgsecret! to
[task 2019-07-26T01:46:37.027Z] 01:46:37 INFO - GECKO(2216) | MEMORY STAT | vsize 20974919MB | residentFast 1031MB
[task 2019-07-26T01:46:37.029Z] 01:46:37 INFO - TEST-OK | browser/components/aboutlogins/tests/browser/browser_masterPassword.js | took 90269ms
[task 2019-07-26T01:46:37.030Z] 01:46:37 INFO - checking window state
[task 2019-07-26T01:46:37.031Z] 01:46:37 INFO - GECKO(2216) | JavaScript error: resource://testing-common/PromiseTestUtils.jsm, line 112: uncaught exception: Object
[task 2019-07-26T01:46:37.033Z] 01:46:37 INFO - TEST-START | browser/components/aboutlogins/tests/browser/browser_openFiltered.js

Mass move to about:logins

This test is currently passing on Linux as of Nightly 85.0a1, and passes on Linux when running in verify mode. Going to go ahead and mark this as fixed.