FireFox 21.0 for Mac Captcha Validation errors

I had a client report to me their Firefox on their Mac OS kept getting validation errors after submitting the form and captcha, and they switched browser and it worked. I have a Mac as well, and I was able to duplicate this problem with Firefox 21.0 . FYI and any help is appreciated. Thank you

http://ww.wp.xz.cn/extend/plugins/really-simple-captcha/