Warning: Unresponsive script

I keep getting an error message when I launch the site, but only in Firefox.

I have the latest version of the plugin.

unique-coupons.com

Warning: Unresponsive script
A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete.

Script: http://unique-coupons.com/salem/wp-content/plugins/dynamic-content-galler-plugin/js-mootools/scripts/mootools-1.2.4-core-jm.js:12

https://ww.wp.xz.cn/plugins/dynamic-content-gallery-plugin/