It looks like a Firefox bug. It works fine in Chrome, and if you change the action to append some text to a text object instead of alerting, it works fine in Firefox too. I guess Firefox gets confused about mouse clicks when it shows an alert in response to a click. If you hold down enter for a few seconds (so it's OKing the dialog constantly) it eventually goes away too... very odd. I'll look in to it, but I'm not sure I can fix this one.