There are 3 tests in the testing/web-platform/tests/xhr/ directory that are expected TIMEOUT:
/xhr/abort-upload-event-abort.any.worker.html
/xhr/abort-upload-event-loadend.any.worker.html
/xhr/open-url-multi-window-4.htm

I have verified these locally. the about-upload-event* tests didn't stay open, just timed out. I think it is because this is a test that runs multiple subtests even if it times out on the first one.

the /xhr/open-url-multi-window-4.htm test timed out on this case:
XMLHttpRequest: open() resolving URLs (multi-Window; 4; evil)

I didn't see anything in the console to indicate a JS error or something else.

:nhi as a triage owner for this component, can you help find someone who can determine if these tests are timed out because the needs need updating, there are bugs in firefox, or features that firefox doesn't support [yet]?

Leave this open since I haven't figured out why /xhr/open-url-multi-window-4.htm is failed.

Backed out for wpt failures on event-timeout-order.any.worker.html

Backout link: https://hg.mozilla.org/integration/autoland/rev/fca2a64739b13da32d93ee5d48f6bd2e2f23fadc
Log link: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=305801841&repo=autoland&lineNumber=5027

Backed out changeset ed1fd081cc06 (Bug 1642193) for causing wpt failures CLOSED TREE

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&selectedTaskRun=XvRu7aaiRsq8YTP75o-NdA.0&resultStatus=testfailed%2Cbusted%2Cexception&revision=ed1fd081cc06d9630a29277506bd6bf13f4d3912

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=307077601&repo=autoland&lineNumber=5618

Backout: https://hg.mozilla.org/integration/autoland/rev/9432b16790667aaeb603fa66fc34a196e4606655

https://treeherder.mozilla.org/#/jobs?repo=try&revision=2698a0f30540ded877afb89790d7d981afe92ec2&group_state=expanded

https://hg.mozilla.org/mozilla-central/rev/afddfb994fcf

The leave-open keyword is there and there is no activity for 6 months.
:jstutte, maybe it's time to close this bug?

There is still one test open-url-multi-window-4.htm needs to be enabled.
I'll try to do this in this week.

Let's close this bug and file a new one for open-url-multi-window-4.htm.