Make associate/disassociate work in Firefox too.

The solution in the previous commit, although more sane won't work
in Firefox. The problem is that Ember does not pass the event object
and although Chrome copes with that, Firefox doesn't.

Check:

http://stackoverflow.com/questions/15757193/event-is-not-defined-in-firefox

and

http://blog.strictly-software.com/2009/10/windowevent-support-cross-browser.html