It happens again here on Discourse, on my forum that is still in version 1.9.0.beta12 +21, updated after the fix made by @eviltrout, it continues to work.
The problem is on Firefox 56.0.1, no issue with Chrome.
AFAIK, it has to do with the way Firefox works with the clipboard. I don’t know if the security fixes have become stable enough to make working on it worthwhile, but I’d hope so.
Something about the double/triple-click selection which makes the selection go past the boundaries of what’s selectable and thus preventing the quote button to show. We could use another css selector but that would mean displaying the quote button when you highlight the buttons under the post…