Hi tk,
Thanks for the suggestions. The site is http://www.idaholandlordlawyer.com. I took your advice and deactivated all other plugins except the MailChimp Subscribe Form. It did not fix the problem. Weirdly, when everything else was deactivated, the form also did not work in MS Explorer, but worked in Google Chrome. Once I reactivated all of the plugins, it went back to its original behavior – works in Explorer & Chrome, but not in Firefox.
Thanks for the help.