Optin cat sometimes not redirecting to thank you page on Firefox (Mac and PC)

I have a site (www.consciousculturegroup.com) that has the free version of the optin cat plugin.

Although my initial tests seemed to work fine, now (during a final test for the client) the plugin is accepting my email address but (sometimes) not actually redirecting to the thank you page although it is showing a green bar above the email field with the URL of the thank you page, very strange.

We tried it out on another fresh Mac and it worked fine on all 3 browsers but a fresh PC just failed (as above) on Firefox. Note that in all cases the email is pushed properly to Aweber.

Please reply as soon as you can as we are ready to promote the site and need to get this once last thing working before that.

Thanks so much,

Ed