Thanks for writing us back. In testing the form on my end in Firefox, I was seeing that both the Confirmation Email and Success Message were being received as expected.
Can you confirm that there are no active extensions on your Firefox browser, and also try clearing your cache and cookies, then trying again?
Also, when you click Subscribe, are you receiving a Confirmation Email at the email address you're signing up with? If you are, it's an indication that the form itself is working, but the success message isn't showing up on your end. When testing it out, make sure to click Subscribe once, then give it a few moments to see if that Success Message appears after waiting a bit, as it may just be taking some time to generate it.
If you have any further questions or have any concerns after troubleshooting the above, feel free to post back here and I'd be happy to help!