I have had no problems at all with Firefox and Citicard on the Mac, even after their update.

Try going to Preferences:Privacy, click on "Show Cookies" , find cookies with the words "citi" or "accounts" and remove them. Also click on "Exceptions" and be sure none of the citicard or accountsonline sites are being blocked.

When the say they don't support Mac it often means they just don't understand it and can't answer your questions