It has been tested to work in Chrome and Internet Explorer and on a few mobile web browsers with no issues.  

When a user submits an optin while using Firefox they end up with the following message:

Problem in optin!

An unknown error occurred. Please try after some time. 

What is going on and why is this happening?

Optin code can be found at the top of the webpage.

Thanks for the help and your fast response is appreciated.