We have PerformanceEventTiming enabled in Nightly for over a month now and we've fixed all the regression. So I think we can enable it in all channels now.

https://hg.mozilla.org/mozilla-central/rev/cc010190a1ff

Release Note Request (optional, but appreciated)
[Why is this notable]:
This is a new API for web developers
[Affects Firefox for Android]:
Yes
[Suggested wording]:
Event Timing API is now supported.
[Links (documentation, blog post, etc)]:
Spec link: https://wicg.github.io/event-timing/
MDN: https://developer.mozilla.org/en-US/docs/Web/API/PerformanceEventTiming
Implementation bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1667836

Sean, you requested an addition to our consumers release notes (https://www.mozilla.org/en-US/firefox/notes/) in addition to the request to have it added to our developers release notes (https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/88).

We have a Developer section in our consumers release notes for tooling and platform changes that are impacting enough for web developers to be broadcasted more largely than through our notes in MDN. Does it fit this category? Thanks

Yeah, the developer section fits well, thanks!

Note added to 89 nightly release notes with this wording and a linki to MDN, thanks:

The Event Timing API is now supported.

This is in our 89 beta notes and should be in our final notes managed by our comms team.

MDN has been updated for this feature; see https://github.com/mdn/content/issues/4309 for more details.