It's a feature of X (so works in any app running in X, not just FF and your terminal emulator). Wayland changed clipboard behaviour (along with many other things) and decided the selection clipboard was too large a security hole to keep. It's pretty useful though, so Gnome added it back, on top of Wayland. It's still a large security hole though!
Separately, PuTTY has a similar mechanism for copy, though this goes in the normal Windows clipboard.
Separately, PuTTY has a similar mechanism for copy, though this goes in the normal Windows clipboard.