The title is kinda misleading. "[W]ith user's permission" probably should be "with user's action" instead.

By default, you can copy to clipboard in all the major browsers when user takes an action, like click some buttons or links. So all these "copy code" buttons won't stop working. What does not in Firefox (and shouldn't in Chrome) is to do so without user's action.

Note: I have no idea what the rigid definition of "user's action" is. But clicking a button counts.