The issue of the Quote button not working when triple-clicking to select an entire paragraph in Firefox is being discussed. dax reports that the problem persists despite a previous fix, and it only occurs in Firefox, not Chrome. schungx confirms that the issue is specific to triple-click selection, as manual selection works fine. zogstrip notes that the problem is not easy to fix, but schungx and Mittineague speculate that it may be related to Firefox’s clipboard behavior or the way triple-click selection affects the selection boundaries. zogstrip suggests that using a different CSS selector could be a potential solution, but schungx points out that the Quote button does appear, but clicking it has no effect.
Continuing the discussion from Quote not working if I get the whole sentences:
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.
You mean triple-click to select the entire paragraph.
When the entire paragraph is selected via triple-click, pressing the Quote button does nothing.
When the entire paragraph is manually selected, pressing the Quote button works.
Still occurs on Meta as well as latest build.
This is true but only with Firefox. On chrome everything works well (triple click and select manually a sentence).
I will update the title
Unfortunately it’s not an easy problem to fix. We’ve tried multiple times to fix it but it keeps breaking for some reasons…
Feel free to take a stab 
at it though 
(pr-welcome)
What has been the problem preventing the quoting?
I suppose if the quote button appears, it is a matter of checking what is included in the selection that throws it off?
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…
Well, thing is, the Quote button DOES appear and is clickable.
Clicking on it merely does nothing.