It turns out I had privacy.firstparty.isolate enabled when I saw this. It would be nice to fix compatibility with first-party isolation for Tor Browser's sake.
Bug 1647829 Comment 1 Edit History
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It turns out I had privacy.firstparty.isolate enabled when I saw this. It would be nice to fix compatibility with first-party isolation for Tor Browser's sake. (Thanks to Johann for spotting this.)