I seem to be having the same problem, but a different cause. I'm not using Featured Content Gallery, but mootools for an image slideshow. I get the error
uncaught exception: [Exception... "Could not convert JavaScript argument" nsresult: "0x80570009 (NS_ERROR_XPC_BAD_CONVERT_JS)" location: "JS frame :: /wp-content/themes/inove/js/mootools.js :: anonymous :: line 2004" data: no]
On line 2004 of mootools there is: if (this.addEventListener) this.addEventListener(type, fn, false);
I have no knowledge of javascript whatsover, but I've noticed that if I remove base.js and menu.js, the error is gone. So obviously, it's these javascripts conflicting.
Since neither of them are plugins (but I integrated the slideshow in the theme), I can't use ericmann's solution. Can anybody help me? I really need both scripts.