I just did a Try run with some Test-Verify tasks for (slightly-modified) test_bug1261673.html, and I got this failure back from one of the tasks:

TEST-UNEXPECTED-FAIL | /tests/dom/html/test/test_bug1261673.html | changed preference: doh-rollout.doneFirstRun

Here's the log with the failure:
https://treeherder.mozilla.org/logviewer?job_id=413088341&repo=try&lineNumber=3986

This test does not in fact toggle that pref (or have anything to do with that pref); instead, this pref apparently gets toggled in the background, lazily, on the first time we do some DoH (DNS-over-HTTPS) activity, or something like that.

So: this isn't really a legit test failure. We should add this pref to testing/mochitest/ignorePrefs.json

This pref gets toggled lazily in the background, without tests explicitly
setting it, so it's appropriate for us to ignore it when auditing for prefs
that have been persistently modified during a test run.

https://hg.mozilla.org/mozilla-central/rev/88638acd2532