How to copy data to the clipboard with Greasemonkey?

I found this question but trying to use the code given there:

unsafeWindow.netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
const clipboardHelper = Components.classes["@mozilla.org/widget/clipboardhelper;1"]
    .getService(Components.interfaces.nsIClipboardHelper)
;
clipboardHelper.copyString('test');

Gives the error message:

A script from «http://example.com» was denied UniversalXPConnect privileges.

I also tried to use (from Mozilla site):

const gClipboardHelper = Components.classes["@mozilla.org/widget/clipboardhelper;1"]
    .getService(Components.interfaces.nsIClipboardHelper)
;
gClipboardHelper.copyString("test2");

Which gives the error message: Components.classes is undefined.

In both cases it doesn't work with the latest Firefox on Windows 7.
What else should I try?