Christoph, is this a Content Security bug? Kannan found this problem as part of his nsIDocShellTreeItem removal analysis for Fission. I don't know how this bug affects Fission users. Tracking for Fission Nightly (M6) because this bug doesn't need to block dogfooding.
Bug 1588178 Comment 4 Edit History
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Tracking for Fission Nightly (M6) because this bug doesn't need to block dogfooding.