Well, it's a browser option, but my browser (Firefox 39.0.3) is already set to "Open New Windows in a New Tab" and it doesn't work when I click on the Reply link.
That's because that setting only instructs Firefox to open a new tab instead of a new window
in instances in which a link is written to open a new window when clicked. If the page source code does not specify that the link open in a new window, it won't, unless one of the keyboard/mouse shortcuts described above is employed.