As a firefox user, that copy/paste drives me insane, but I think you're making a logic leap here along with assuming the worst. It's very possible (and likely) that there are API deficiencies that break their ability to offer this when overriding the right-click menu. For example, Firefox not allowing javascript in random tabs from writing to the system clipboard.

You can still use Ctrl+C / Ctrl+V (and in fact the UI will (or at least used to) tell you that).