I don't think it's related to bug 1562667 because we haven't supported/changed the testing code path, IIRC. But it's related to SAB and postMessage, I will take a look.
Bug 1593526 Comment 3 Edit History
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I don't think it's related to bug 1562667 because we haven't supported/changed this testing code path, IIRC (See https://bugzilla.mozilla.org/show_bug.cgi?id=1589673#c6). But it's related to SAB and postMessage, I will take a look.