Bug 1068479 - Copy paste toolbox has no hit state

https://bugzilla.mozilla.org/show_bug.cgi?id=1068479
This code pattern is very dangerous; people might hit the button twice and we might call `hide()` twice and your event listener will be registered twice.

The current code below is harmless in that situation but it's better if we could use `handleEvent` pattern correctly for the first time (than rewriting it later).