Web site won't let me add a payment method

I'm not sure- I know a few people have been having issues with the site as of late.  You said you're using Firefox, did you try clearing the cache and cookies?  What about trying out a different browser?

I'm terribly sorry that you spent the time trying to reach out to messaging and they could not help.  You might want to look at reaching out T-Force via Facebook or Twitter (after you've cleared the cache/cookies and tested another browser) to see if there's something wrong with the website and if a ticket needs to be filed on your behalf.