It's not up to Firefox to support add-ons, but rather for add-on developers to keep up with new versions of Firefox. This page - http://www.discovercard.com/customer-service/security/soan-download.html - says that add-on is compatible with Firefox 3, under System Requirements on that page, which indicates to me that either that support page hasn't been updated in a few years or that add-on hasn't been updated in a few years. Check with Discover support to see what the status of that Secure Online Account numbers program is, as far as Firefox compatibility goes. <br /> Firefox 3 was released on June 17, 2008 - 3 years ago <br /> Firefox 4 came out on 03-22-2011, and Firefox 5 is sue to be released this coming Tuesday June 21.