As far as I can see, this works now, but the behaviour of forward-deleting into a quote has changed. It seems to delete the <blockquote> element when before, it didn't.
Bug 1581337 Comment 9 Edit History
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As far as I can see, this works now, EDIT ~~but the behaviour of forward-deleting into a quote has changed. It seems to delete the <blockquote> element when before, it didn't.~~ Scrap that second part. Looks like forward delete into a quote hasn't changed. The behaviour depends on the content of the quote.