Opera 25 on the mac fires once and Firefox 31 on the mac both fires twice. I tried to reproduce this issue by running

JX.Stratcom.listen('focus', null, JX.log);

in the console for each browser. Then I clicked into the browser window. The 'focus' event is then fired 1x and 2x for Opera and Firefox, respectively.

I think the issue for Firefox is from here http://www.quirksmode.org/dom/events/blurfocus.html

Firefox fires two focus and blur events when the events are registered in the capturing phase.

So I think the fix is to not register these events in the capturing phase for Firefox. Another web link offers thoughts on how to do this:

You, the web developer, can choose whether to register an event handler in the capturing or in the bubbling phase. This is done through the addEventListener() method explained on the Advanced models page. If its last argument is true the event handler is set for the capturing phase, if it is false the event handler is set for the bubbling phase.

from http://www.quirksmode.org/js/events_order.html