Bug 1660968 changed the loop to go through each window context and checking if document is available. With Fission it is. I assume in comment 9 FF wasn't restarted after changing fission pref.
Bug 1748005 Comment 13 Edit History
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Bug 1660968 changed the loop to go through each window context and checking if document is available. With Fission it is. (Aug 2: I think I meant 'isn't') I assume in comment 9 FF wasn't restarted after changing fission pref.