As you may be aware the firefox Promise implementation has not been using proper microtasks and we're trying to fix that. Part of that effort uncovered an issue with a test for async waitUntil():

https://bugzilla.mozilla.org/show_bug.cgi?id=1409979

Apparently the spec is correct, but the test follows an incorrect interpretation based on our bogus microstask handling. Chrome matched the test expectation by doing this workaround:

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/serviceworkers/WaitUntilObserver.cpp?l=145&rcl=c2665c0a9bfec6379c90d3ea36199ee6a3e9f024

We would like to correct the test to follow the spec and real microtask definition, but wanted to make sure everyone was ok with that since chrome matches behavior.

We think this example from the original issue is incorrect: #1039 (comment)

And this WPT test condition should return OK and not throw an InvalidStateError:

https://github.com/w3c/web-platform-tests/blob/a16738c524a65946b0ff7df6be37ae076ff5e860/service-workers/service-worker/resources/extendable-event-async-waituntil.js#L20