Turned out this is simple, and checking the `currentWindowGlobal` is enough. If the frame has been closed its value will be `null`. Here a try build: https://treeherder.mozilla.org/#/jobs?repo=try&revision=aab88e9b507542f412ba33ff5330ddd9971d14db Something we would still have to check is how to get the new browsing context id.
Bug 1666204 Comment 9 Edit History
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Turned out this is simple, and checking the `currentWindowGlobal` is enough. If the frame has been closed its value will be `null`. Here a try build: https://treeherder.mozilla.org/#/jobs?repo=try&revision=aab88e9b507542f412ba33ff5330ddd9971d14db Something we would still have to check is how to get the new browsing context id, but that's for a follow-up bug.